yes - at those bigger "pizza by the slice" joints anything more than 2 slices for lunch will be driven by pizza-induced greed and I definitely won't get any work done afterwards. At dinner post-work I can maybe put away 4 slices...
Almost all the by-the-slice places I've been to take slices from their large or extra-large pies. The only exception I can think of would be Pinup Pizza on the Las Vegas strip (30 inch diameter pie).