I think we're pretty similar in size, I'm a bit taller though (6'1"). I don't have any waffle knit heattech, but I'm pretty consistently medium across the board in Uniqlo too.
Just put the extra warm heattech back on, noticed a minor difference in fit: the sleeves are longer, more like regular length than the shorter length sleeves on normal heattechs.
Extra warm heattech came. Fabric feels like a cross between regular heattech and fleece.
Warm and comfy as hell. I've been wearing a regular heattech shirt and fleece jacket while bumming around at home, but this thing alone is just as warm.
Wish they had v-necks.