I have the ass the size of Manhattan and even I find some of the underwear in Large to be a bit roomy. I must have 10-15 pairs, so I'd say sizing is pretty consistent. The striped ones fit great; the solid ones are good, too, but seem to stretch out a lot more. I wouldn't size down. If you're feeling the need to "size up," you're probably not being completely honest about your "true size."
I think I'm allergic to heattech... it left some weird ass blotch/bruise on my thigh
4. Uniqlo Heattech thermal clothes contain milk – Japanese company Uniqlo mix rayon and milk protein to create a unique system which traps moisture enabling the garment to retain heat.
Read more here: http://www.esquire.com/style/uniqlo-heat-tech-series-1208#ixzz1lK71fex0 and be careful which thermals you buy. I think these thermals are only available in Japan and America but watch out! I’m not sure if this is stated on the label.
I tried the medium, bought 10-12 if I remember correctly. Of course I ordered online and later found out could not return. I ended up burning them
That said IME, they did not fit TTS. Perhaps I needed a large, but I think I would feel a lot more comfortable in size XL. More so, due to the low rise, which seems persistent throughout the cuts, sizing down / TTS is not good IMO. YMMV