Size doesn't seem to be the only factor. Miami is 8,000 square feet, La Guardia is 5,300. Lots of complaints about crowding at MIA, but few at LGA. Heathrow Terminal 3 is well served by other lounges, which should reduce the demand for access to any one lounge. In any event, I expect American Express had to go after a space that was available, and they have demonstrated a willingness to expand when they can (DFW, MIA, SEA).