Who would be stronger, someone with a lean body or someone with a lot of muscle mass? Why?

Size doesnt matter , what matters is the type of workout each one is doing.

When you do 8-12 repetitions per exercice you are training your strength and the muscle will get bigger and this called sarcoplasmic Hypertrophy while when you go heavy between 1-6 reps you are training your fast twitch muscles which will generate more power and less size and that is called myofibrillar hypertrophy.

If we refer to the term strong to power which is the max speed of lifting a certain weight Power =( Force x Distance )/time the person who does more repetitions for the same time is the stronger one coz he can do more work.

After all it all depends on each ones sport .Usain bolt is stronger than Ronny coleman in running but coleman is stronger in lifting weights.

A lot of factors are effect on ones strength , it is directly related to the client diet and way of training, but normaly having more muscle and relative fat amount  would able you to be stronger .

muscle mass is more stronger becouse streght is depend size of muscle cells.lean body people hav good flexiblity and stamina .but muscle mass people have heavy streght and power ex,power lifters,weigt lifters

Key Points:

1. A ton of factors influence strength beyond muscle size and skill with the movements used to test strength.  The strength of individual muscle fibers, normalized muscle force, muscle moment arms, and body proportions can all have significant, independent effects on strength.
2. Just as there’s massive variability in muscle growth – some people gaining a ton of muscle in response to training, and other people gaining very little – there’s massive variability in strength gains as well.  Normalized muscle force (how strong a muscle is relative to how large it is) can increase up to 39% for some people and decrease by as much as 5% for others, in response to the exact same training program.
3. Early on in training, there’s a very weak relationship between gains in muscle and gains in strength.  Gains in muscle mass may explain as little as 2% of the variation in strength gains for new lifters.
4. For more experienced lifters, gains in muscle mass may explain up to 65%+ of the variability in strength gains, highlighting hypertrophy as a key factor for strength gains in trained lifters.
5. Training style has a big impact on the ratio of strength you gain relative to size, with heavier training generally producing larger gains in strength.

6. If you’ve spent much time in the gym, I’m sure you’ve witnessed a scene very similar to this one:

   There’s a big guy in the squat rack busting out reps with 405 lbs, making each rep look like a herculean effort.

   In the rack right beside him, there’s a smaller guy with smaller legs repping the same weight, making it look like a piece of cake.

   If you haven’t seen it on the squat, then you may have seen it with the bench press or deadlift.  And if you’ve never seen anything like this – trust me, you will at some point.

   At first, this may seem confusing.  After all, muscles produce the contractile force to move weight, so the guy with more muscle should be able to move more weight, right?  How can a guy who only weighs 163lbs deadlift 684lbs, when there are plenty of much larger men who would be 

perhaps the lean one , because the job does not need a muscle and it needs a stronger mind not muscle.

When you talk stronger you talk muscle mass!

Someone with a lean body mass. because he will be more easy to move but he still has muscle

It all depend on attitude. An attitude towards something defines your persona.

   It all depends on whether the guy with lean body is doing something or not. For example, if this is a lean person is a master of martial arts or a world champion in boxing, then he will beat the guy with big mass.   If a thin guy does not do anything, then he will beat him.   But if they are both masters of martial arts or world champions in boxing (of course, in different weight classes), then only fortune  will be decide, because if a person with a large mass gets into the lean person, then he will kill him, because he has a blow Stronger, but the lean guy has a big advantage - it's speed, due to the fact that he is thin, he can make a person with a large mass be exhaust.

due to atrophy and hypertrophy

First of all, You need to undstand that:

1) people with a lot of mass, mean that the Body structure are bigger than people with Lean body, So...  Bigger mass means "MORE" Contractile cells, and automatically more Strong

2) people with lean body mean that the Body Structure are less bigger, and also the less structural waight requise less Mass. And also, less mass means "LESS" contractile cells, and less strenght

BUT WAIT!

This is a scentific and genetic condiction that affect EVERYONE! But doesn't matter how big are the mass at the beginning, because everyone can increase it!

Everytime you workout, or you do sport, lift weight, ecc...

You get some damage at all the contractile cells, and everytime these cells will be repaired and resupplied with a bit more cells, that's how the Volume of the muscle increase!

What does it mean? it mean that via increasing your volume, you will automaticaly increase your max limit, becomeing stronger than before!

However, muscle can not increase more than% of the original size, but you will get stronger anywhere, because when you reach the max size, u can focus on the technical part of the exercise that keep you more stronger even if u don't get more Volume...

To explain perfectly, i need to talk for3-4 hour, and this post it's only a "broad explanation", however, hope it help you!