Thor doesn't even need to be "stronger" - I think it's reasonable to assume that the Hulk may actually win a battle of PURE strength. But focusing solely on physical strength completely demeans the livelihood of Thor. Thor's strength is only partially physical; in essence, his greatest assets are his worthiness of the Mjollnir, and the godlike weather control and flight and other such awesome things.

Hulk can't pick that shit up.