Yeah I've found Sainsburys are considerably bigger. As are Tesco. DS is also long in the body but really skinny too.
I've stopped buying Boots, Asda and Morrisons because the sizes were just so out compared with other places. The 3 - 6 month vest tops from Sainsburys were a similar size to 6 - 9 month ones in Morrisons. That's when we gave up with them.
Its a massive pain in the backside that sizes differ so much as we buy in the sales like you. Which is a shame as there is some nice Nutmeg stuff and I prefer it vastly to Tesco's range.
I'm finding Mothercare, Tesco, Sainsburys, Gap, M&S match his age better at the moment. Annoyingly the brand he fits the best is Polarn O Pyret which he got for Christmas. Which is bloody typical. Tesco, is probably second best though.
I don't have much Next stuff, although he does have a Next hoody which is lovely but tight to get on and off.
We've had to buy braces to keep his trousers up, as he's too skinny for virtually every pair. I think he has a pair of joggers from Sainsburys that don't end up around his ankles in five seconds flat. Dungarees are the way forward we've found.