Yes i agree with you Doublefreud i remember in Gothic 2 a big troll guy somewhere at the edge of map which was impossible to defeat face to face at that time when i discovered it but it just happened so that i had shrink spell and i used it on him and wow i killed it and got tons of xp and i really felt good :P. Ofc this spell was very rare and u couldn't do this to every enemy but still u "outsmarted" it at low lvl and that shouldn't be punished with less XP just because u are low lvl and u aren't supposed to kill such enemies.
Thats why i think enemies should have fixed amount of XP. Also when u are exploring some area u might overlook some dungeon or smt and when u figure this out at some higher lvl and come back u will feel that u been robed of that xp cuz u didn't do it back then.
Another thing that scaling XP u get from enemies is doing is forcing u in linear gameplay and that is smt i really hate in RPGs especially RPGs that are build open world like but arent cuz of this.
For example lets say in Baldurs Gate u forgot to go to Firewine Bridge (a big labyrinth at the bottom of the map) and when u get in the city Baldurs Gate (late ingame) u decide to go down to that map and u figure out whats going on. U find out that its full of low lvl creatures but its hard as hell because of all the traps big numbers of enemies from everywhere and now wouldn't u fell stupid to not get xp from killing those pesky little creatures even at high lvl when killing such creatures should be easy? If xp scaling would be implemented and u knew what to expect would u even consider doing it?