As stated by others, there is a "standard" amount of XP per critter of X rank and Y level, but that standard amount can be modified by a number of factors. "In the wild," certain critters or factions are given added or reduced XP. For example, Family as a group gives reduced XP at 50 as they were considered less dangerous as a whole; Rikti portals used to give XPs but that was removed and the Lts that summon them given bonus XP to compensate; and as stated above, Freaks has a sightly lower XP rate that is balanced by the few critters that rez for "bonus" XP.
The AE also has XP modifiers for custom critters. They have to have at least one ranged attack and enough attacks or defenses to make them "worthy" of 100%. Additionally, custom critter factions need to have a mix of bosses, Lts, and minions to get full XP as the OG devs wanted to curb the appeal of Boss and EB farms.
Per Wiki:
Level
Minions
Lieutenant
Boss
50
2866
5732
17196
https://hcwiki.cityofheroes.dev/wiki/Experience