Somethings funky. Why no backpedaling with the 18 (which would be easier), and backpedaling with the 16T (which would be more difficult)?

I like to ride low gearing and just spin like a mother ****er. It's more comfortable, It's faster off the line, and generally a little quicker except for in flat out drags.

But I suspect your problem is bad form, and weak off season muscles. Not gearing.