Why are vinyl window frames so bulky?

Replacement window frames, vinyl and aluminum, are indeed more bulky than new construction window frames. New construction windows are usually built to hold extremely light weight “single strength” glass and include a “nailing fin” along the edge of the frame that adds strength to the frame. Replacement windows have no new construction “nailing fin” and the frames must be strong enough to hold heavy-weight energy efficient glass. Of the two types of replacement windows, it is true that vinyl frames are slightly more bulky than aluminum frames, obviously because vinyl is not as rigid as aluminum.