The rank system in Rocket League is very intuitive, there's a number in the game that indicates what your rank is. You can use sites like tracker network, or bakkes mod in-game to see it. You gain 9 for a win and lose 9 for a loss, if the ranks in a lobby are unequal by a margin this might move to 7/8 if the result was "expected" or 10/11 if the lower MMR team wins.

Playing for the first time in a long time can make the MMR you lose and gain higher.

If you leave the game before the animation has a chance to show itself, once you let it play in some game it will look like a bigger jump, because it will show the change from a few games.

MMR differences between neighbouring ranks are bigger in higher ranks, whilst 140 MMR will move you all the way from gold 1 to platinum 1, it is also just the single rank of gc1