Justifying the small size of a dataset due to collection difficulties
As has been said, a satistical analysis of your data set will clarify with how much confidence and accuracy you can  conclude your findings. This an essential part of the scentific process

In the natural sciences particularly, data collection can be problematic and limited by time and budget,  but the community are aware of this, as should be your reviewer. 

All that is required is that you report your findings appropriately  even if you don't necessarily like the answer! In extreme circumstances you may have data sets that are so scant or varied you simply do need more work. It happens.

Science progresses with knowledge which includes less than optimal data,  as does helping those that come after you do better by explaining in your paper how you could improve your own approach. 

I agree the reviewer is probably asking for this kind of analysis and not why you were unable to do as much work as you would have liked