(it's their main item and I guess most people get it),

I suspect this assumption is pretty far off. I'd guess the single burger doesn't even crack the top 10 in sandwich sales for them. Not even close if you exclude sales to kids. At least in the US.

If for arguments sake a more typical lunch was 1/4pounder w/cheese, medium fries, medium (regular coke) you are something like 1100 calories - well into heavy lunch territory.