Originally Posted by
Badger6
I find that my rear is easier to lock on my road bike (140mm), where on my gravel bikes it requires a lot of deliberate action (160mm). YMMV, but I find the smaller discs have a much narrower modulation threshold. I think the smaller discs are for weight saving, because some people think saving 20 grams (less than 1 oz) matters.
Google 'leverage'. It's a simple concept. Smaller rotors are going to create less braking force all other things being equal. That's why pretty much everything w/ disc brakes uses smaller brake systems on the rear compared to the front. It's all about putting the power where the traction is. Less weight is a nice byproduct of this but not the main reason for it.