there's more than just the main triangle that gets tighter & more rigid-the seatstays become shorter making the rear triangle come closer to that of an equilateral construction in both the vertical and the lateral.
if you study a pic of my *semi-compact* steel cinelli, you'll be struck that the rear triangle comes close to forming a classic pyramid structure.



the toptube length becomes shorter as well,thus more rigidly locating the steerer/headtube so that there's less torsional & lateral deflection when force is applied to the bars or from steering inputs & cornering deflections.

in spite of the *potential* for some loss in rigidity from the saddle to seatcluster due to the need for a longer seatmast, the vertical structure from seatcluster to bottom bkt and the longitudinal structure from the headtube to rear axle dropouts & from seatcluster to headtube is dramatically more rigid & better braced in nearly every deflection mode, enuf to easily offset your long, flexible seatmast concerns and which in addition, can be partially or completely addressed by reinforcing/stiffening of the seatpost itself.

to me, the real question is whether or not all the added stiffness in a compact design is necessarily consistantly beneficial and without trade-offs . in my experiences, the thorough, well-considered, goal-oriented implementation of any given design philosophy is the real key to any superior and satisfying result.