As a business development manager in a growing business, is it more of an opportunity or a risk to approach big-brand clients to add to client bases?

Big clients are more business ppotential and thus could be taken as more challenging opportunity.

Big clients means big business. Do not leave the opportunity.

It is easy to speak of big brands easily. Big brands would never come into you if you don't have anything.

For example, if you have a user review system and want brands to answer or redress issues their clients have, you could first create a portal where people can post their grievances. Once you have enough reports you just need to prove most of them are genuine ones.  If there are more activities in your portal, now take the solution to the brand. They will surely hear you as you have a ready to use solution for them.

You need to see how you could implement these into your Business Plan