I think that's how EXP worked in BW, but obviously it was changed because
EXP has been the following...
Gen 5 had you gain Scaled EXP depending on level. Lower level mons get more EXP, higher level mons get less.
Gen 6 revised the EXP share to it's current form and removed EXP Scaling, this is probably why you had multiple level 40 pokemon by the time you got to Korrina. You also gain EXP on capturing pokemon now as well (I don't mind this addition)
Gen 7 brought back the EXP Scaling system, but it's scaling not strong enough to account for the 250% more EXP you get per pokemon. No problems though, you can turn the EXP Share off if you wish for something more traditional.
...Until Let's Go which removed the ability to do so.
Gen 8 merged the Happiness and Affection Stats into the one new Friendship stat. One of the buffs you get at the higher level of Friendship is 1.2x EXP per battle.
In Sw/Sh, there is a cap on Friendship you can earn without using special methods like the Pokemon Camp, so the buffs were optional for the most part.
In BD/SP that cap no longer exists, so you can get all the Friendship Buffs like the boosted EXP, dodging moves, surviving moves that would KO you and such just by leveling up and walking about.
I'm sure there's extra stuff in there (I only just found out about boosted EXP if you pokemon can evolve last week) but that's the main gist of things.
