Who says a compact frame is stronger? Tensile strength and tensile stiffness are two different things. The shorter the tube, with a given diameter and wall thickness, the stiffer. Round tubes are stronger than oval tubes in all directions. An oval tube is stronger in 2 directions but weaker in the other 2 directions with a given circumferance and wall thickness. Steel tubes of a given strength or stiffness have a thinner wall. Aluminum will bend, depending on the alloy, but it will not bend as far or as many times before failing.

Compact frames are lighter and stiffer because there is less mass and the tubes are shorter. Aluminum frames tend to be stiffer because larger diameter and thicker walled tubing is used (in most cases). Remember the early Vitus frames..... flexy and made for a nice ride in my opinion. They had used small dia tubes.

Seat post has no bearing on frame stiffness. It is an external part and since most of them are alloy, they had better not bend much or the rider will find himself quite uncomfortable.

You also need to consider the manufacturing process. Forging vrs. casting. Forgings will bend more without failing than will a casting. Castings are strong and stiff but brittle and will fail abrubptly but not as strong as a forging. A forging is stronger and will flex more before failing.

Any given tube is much stronger in tensile than in compression. Round tubing is stronger in torsion than either oval or square.