The wheelbase ought not to make any difference to how the bike coasts. For a bike with freewheel, tires, wind resistance (and therefore position), and friction, bearing or rubbing, are the agents. You might check to see if something is rubbing or if your wheel bearings are binding slightly (top run of chain will sag when you sit and coast, if a derailleur drivetrain).
Tho' I'm beginning to think age makes a bike coast slower.
Patrick Moore, who just rode to and from downtown again on a non-coasting 72" fixed drivetrain.