Haven't had twins, but yes you do show a lot sooner. Do a google search of how far along + twin belly pics. Ex: 15wk twin pregnancy belly pics
Most show sooner, but I didn't. Usually if your measuring weirdly large they do an early ultrasound, but I didn't find out we were expecting twins till 21 weeks! I've exploded since then tho. I am huge! And I'm only 24 weeks! :)
I'm showing at 15 weeks but when my mum had me and my sister she didn't show for 5 months. I think it just depends as everybody is different.