Bella was in mediums until she potty trained at 3. She went into large nappies (and we were using pockets exclusively at that time, so hadn't used a DnF) for a while around 11-13 months, when she was at her biggest, but slimmed down considerably when she started walking properly and then became a skinny minnie! Larges would have drowned her then, but would fit her now (at 4!).
Jake, OTOH, is 15 months and is very definitely in larges. I'm hoping he starts to slim down a little soon because if he doesn't, I'll have to start looking for XL nappies as he is just chunky. He's over 28lb and has a big bum and legs, but a tiny waist and you can see his ribs.