Most uniform is made in china, or some other cheap place and the model they use for fitting sizes must be the most bizzare looking person ever. So a large one thing, will be slim fit, and another thing, will be baggy as hell. Just try and get what you think, try it on, and return if needed. The folks that deal with it are used to swaps, so don't worry.
I ordered slightly oversized uniform, but for me, my weight goes up and down like a yoyo. I've gone from being quite slim, to a bit chubbier, to what I can only describe as average weight, in the space of about 18 months! I can only attribute my recent not-very-dramatic weight loss to having a string of quite nasty cold/flu things in the winter, which I probably picked up from the railway. And putting on weight was due to swapping to permanent lates which somehow meant I often had cravings for quite unhealthy food at odd times of the night. So some months I fit in my uniform, others not!