in the summer a fair percentage will be away doing overnight trips, and if they don't return until after 6, a number of the nightstoppers will have departed, freeing up stands.
I take your general point about the increase in average pax per movement. However, I think I'm right in saying that Jet2, Easyjet and Ryanair do very little overnight flying although some flights get back after midnight. It's more TOM, TCX and MON that have the late evening departures.
Nevertheless, in summer essentially all the stands are used at some part of the day, even if stands are unoccupied for several hours at various times because as you say less a/c are just parked up. And you can't have stands converted to car parking for part of the day. A look at the apron between 06.00 and 08.00 in summer may well indicate there are very few, if any, available parking stands, with arriving aircraft sometimes having to wait for stands to be vacated. There just seems very little or no margin at all, especially if schedules go haywire during that early morning peak.