Our dd was out of her size ones by three and half months - came up too short on the body resulting in builders bum. She's now just coming up 20 months and we've just ordered size 3's for daytime use as the 2's are giving that builders bum effect again. She's been in size 3's for nights for the last month as size 2's when boosted didn't come up anywhere near high enough and I got fed up of scrapping poo off my wraps, vests etc in the mornings. Have to say the size 3's are huge round the waist on her and cross over at the front but are much easier to get on and sit properly, which is a relief patic for that first morning change.
Relieved to see that size 3 use isn't hugely uncommon as up til recently hadn't met anyone I knew that had needed and was starting to think that either dd was massive for her age (she's nearly off the chart height wise but slim - dh is 6'5" though so not unexpected)or we were doing something wrong.