You may have already looked this stuff up, but to answer the question anyway: you get specific size modifications to your physical stats for pretty much any spell in the polymorph school*. As a rule, shifting to a Medium form gets you +2 Strength, while shifting to a Small form gets you +2 Dex. Tiny forms get +4 Dex and -2 Str (sometimes -4 Strength and/or -2 Con), while Large forms get +4 Strength, -2 Dex and usually +2 Con. Etc. The typical size modifiers to AC and attacks apply, and there is often, but not always, a specific modifier to natural armor as well.
*Note that, if you are not Medium or Small to begin with, you first take some size modifications to "get to" one of those two before you start appying further modifiers. Giants don't get stronger by shifting to Medium forms.