Virtual (touch area size) vs optical element size

I've been trying to improve form inputs in one of my clients app, and I ended with interesting conclusion. I'd like to ask you about opinion on this topic.

Problem is: I have small input fields (number fields) that are quite near each other. I wanted to make it easier to enter one.

My solution: I thought about enlarging them by half (max I could do to keep them separate and cleanish UI), but I thought about alternative approach. I kept their optical size the same and increased virtual size (area that respond to tap) by half. That way I believe user will try a bit more to aim with tap (because they seems a bit smaller) and virtual size aids him with that.

What do you think? Are there any studies on this topic?