How can we make a column more strong without increasing the diameter of reinforcement?

An RC column has 4 important variables to design for a given load, look below for the list:

1. Height of column

2. Strength of concrete grade used

3. Reinforcement bars

4. Column cross sectional dimensions

So, to answer to your question, without increasing reinforcement for a given height, column dimensions would be to increase concrete grade. Many cases this may be workable but you may end-up increasing column cross sectional dimensions as well.