what a great thread!
I think that material the clothes are made with has the biggest impact. For example - Leggings, you can probably get a size smaller than your 'standard' size as they stretch to your shape. Also jeans with a lycra content are going to be more forgiving than a cotton shirt or fitted trousers! It does range dramatically from shop to shop!
I have a large range of sizes, and with designer brands and styles its often hard to work out what a M or a L, or a 3 or a 4 is - and i have to go with what 'fits' rather than what the label states!
We need to be happy MM-ers - cos whatever the size is now, its not the same as when we started on SW (i presume!!) - which is SO much more important to focus on!