Thor is quite used to throwing his weight around in a fight, using his size and mass to his advantage. Aslan is an extremely large Lion, however, and while he may not be quite as dense as Thor, the agility and speed make up the difference to present quite a challenge.

Not quite fast enough that Thor can't keep track of him, but enough to force him to react, to turn and shift his footing so he can keep Aslan in sight, such that when the Lion hits him, he has one foot off the ground. Thor staggers under the full weight of his opponent for a moment before he grabs two fistfuls of that glorious mane and deliberately leans into his fall backward to the ground, pulling the Lion with him and using his foot against Aslan's belly to heave him up and over in an arc.

Thor rolls to his feet as quickly as he can, rushing close to try to pin Aslan down, if he can get there before the Lion gets back up.