Is it easier to paint straight lines with a smaller brush?
Possibly. However...

I would think a 2.5" brush gives you better control, because a thinner brush can result in a more wavy line. With a larger brush, the "waviness" is controlled a little better.

And, a smaller brush can only hold so much paint, and you'll end up with more stops and starts due to very nature of the thing. That can introduce some more imperfection in the process.

Also, you'll end up holding the brush for much longer, and if you're anything like me who can't hold a pencil for longer than five or ten minutes before needing to rest, the entire process will take much more time.

Of course, your mileage may vary.