I have 180 on the front and 160 on the back (hope tech m4) but I am looking for better stopping power. What difference will 203 rotors make.

Will it be significant or marginal?

And other than weight saving why do bikes tend to have a smaller disc on the back.

When you brake most of your mass is transferred to the front wheel, so that needs to do more work. You can improve this a bit by leaning back and down when you brake.

203s give you a bit more leverage on the wheel, and have a bigger surface area so can cool a bit better. Harder to keep em straight though!

Bike radar did some testing on this recently I think and came up with a figure of 20% increase in breaking power going 160-180 or 180-200

and as ooOOoo says bigger discs cool better, but this will only be an issue if you're overheating yours atm

As I don't know your riding style it's just a guess but you can hugely improve your riding by braking less but more effectively. Work on your technique before spending money!

Work on your technique before spending money!

no no no. wrong way around 😉

I've just moved from 160 to 203 on the front and the difference once everything was bedded in was pretty spectacular. Very noticeable power increase and more feel, too. Go for it.

Are your brakes actually working properly? Old skool m4 with 203 front and 185 rear was enough ( if not loads) of stopping power on the tandem)with an all up weight of over 30 stone

makes a massive difference… a cheep worthwhile upgrade… you can never have too much stopping power… it will give you more feel / modulation too…

but the rotors/pads will not heat up properly, so you wont bed in the pads properly and they will wear out really fast.

Makes a big difference, IME, but if you and your bike are both normal-ish weight then 180/160 should give plenty of power – if yr brakes are working properly.

The 200mm set I've used gave loads of power – more than enough for xc stuff – I've downsized to 160 for the hardtail (as it's quite a bit lighted) and use 180 on the fs. So yeah it does make a difference, but only go for it if you really need to.