Weight isn't the only thing that makes flaps more significant. Wing shape and area can make flaps necessary due to the poor low speed qualities of some wing shapes, lengths and cross section. The laminar flow design of the full sized Mustang wing makes flaps manditory due to poor slow speed flight caracteristics while the wing of the Kadet trainer series has so much low speed lift that in a headwind situation, it can actually fly backwards. On the otherhand, Boeing jetliner wings are designed to have the greatest lift within a specific speed range while at low speeds, the lifting characteristics are so bad flaps alone are not enough. To increase lift, leading edge slats were added and the flaps have been designed to slide backwards, almost doubling the effective wing area to maintain enough lift to keep the plane in the air. I'm not going to go into the theory as to why, would get to complicated for some to understand