Support a 5ft long 2x4 with two cinder blocks placed under the ends. Stand on it mid span. Measure distance to ground. Do the same with an 8 ft long 2x4.

Assume we'll make two frames out of straight-gauge tubing one with horizontal TT and one sloping. Now consider that you'll have 4 tubes shortened by using a sloping top tube design, assuming all tubes on both frames meet in a fairly typical seatcluster.

Which one would be stiffer?

Sure, the front triangle would be moving away from the "ideal" equilateral triangle, and on smaller frame sizes, the rear triangle would be, but for larger frame sizes, the rear triangles would be moving towards that ideal.