Oh, no worries. I'm speaking strictly between the 4 current models being sold right now by Apple. See here:
http://store.apple.com/us/browse/home/shop_mac/family/imac/select?mco=MjIwNTQyNjE
Each size has two standard configurations: a base model (cheaper and lower specs) and a higher end model (priced higher with higher specs), totaling to 4 stand configuration models as you see. The top end 27", or the most expensive one in the line up, uses a hotter processor than the rest.
If you're talking about an i3 processor, that's not this generation (they all start at i5s, if you see from the link). So for previous generations, I don't know or can't remember from the top of my head what type of wattage the processors run at in each model. But that said, the same idea still applies: every bigger screen iMac will always run hotter than the smaller screened iMacs, simply because of the screen size.
Hope that helps.