It makes more sense that the speed and maneuverability of the parts would all be greater than the whole. Much less mass to push around. However, being realistic Alpha should have the weakest Hull (being the smallest) and Beta and Gamma should each have reduced hulls that are very similar in amounts. Shield strength for Gamma should be very good considering it has the deflector but shield strength for the other two should probably be weaker than it currently is. (Because they do not have a deflector) Alpha should honestly be the fastest being smallest with the impulse engines right on it (Kind of like a Fighter). It would make more sense for Beta to be more Cruiser-Like.
Those are just my opinions based on how they seems to be configured however. In the show we do not get a very good impression of each piece's strengths and weaknesses. All we see is them shooting phaser beams at enemy targets and Alpha pulling some High Angle of Attack maneuvers.
1) Shields do not come from the Deflector. They come from a grid which covers the entire skin of the starship.
2) I'm guessing the reason the Alpha section has the highest Hull is that it is armored the heaviest (its only connection point to the other sections is in a "divot" under its hull) and it is the thickest section.
3) Ship hulls do not seem to depend on their size. The Avenger and Excelsior retrofit have exactly the same hull, and one is 50% bigger than the other. Likewise, the Defiant refit has exactly the same hull as the Prometheus, and MORE hull than its sections, even though it is 120m (or 150) versus 250m for the Alpha section.
4) Crew IS divided evenly between sections, (150 divided into 50 for each) and seems to more consistently follow ship size than hull does. (The Tier 5 Excelsior has exactly the same crew as the Tier 3, even though it has more hull)
It's my guess that a hull represents not how much damage it takes to completely obliterate the entire mass of a ship, but how much damage it takes to cause a critical failure in one section. (Or even, what it takes to make the Structural Integrity Field fail)
Don't forget also that Hull must be balanced against the damage which can be dealt by foes at the level expected by that Tier of ship. So a 19,500 Hull Constitution refit may be equal in comparison to its foes to a 39,000 Hull Sovereign.