Well, if the 1e progressions were going to match up AT ALL with PC ability growth, then casters would, at all levels, have a MUCH steeper XP requirement than the other classes. Think about it, a 1st level Magic User has ONE level 1 spell. He basically gets one shot to use one spell, then he's kaput for the day. I mean, there is RP and whatever, maybe he can still do SOMETHING, but his only 'class function' is that one spell, period. Even reading magic and such is out, it all requires casting. So his power growth to level 2 is a 100 percent increase in effectiveness. In fact, with 2 spells, he could be pretty effective (though honestly it depends on which spells, so its hard to gauge). Likewise a cleric goes from 3 spells to 4 (nobody has under a 13 WIS, come on), and he gains melee effectiveness by doubling his, pretty good, hit points. Maybe not a 100% increase, but it is pretty solid. The fighter is kind of middle-of-the-road. His hit points double, and he gets a +1 to-hit (if you use the optional rule, we always did). It isn't a 100% increase, but its a 50% increase anyway, almost as good as the cleric gets. The thief. Heh. His abilities hardly change. He does double his mediocre hit points, but with his lousy AC he can't really melee. Even if he back stabs, his damage is the same as level 1. I call it a 35% increase.
So, if we baselined the fighter's 50% increase as worth 2000XP, then the wizard should require 4000XP, the cleric maybe 2500 or 3000, lets say 2750, and the thief around 1750 or even 1500. So we can see that the actual level 1 progression is WAY off for clerics, and maybe a bit generous to Magic Users. It seems like they got the melee classes OK, but undervalue casters. I have NO IDEA where the cleric table comes from, it is just way out of whack!
Things go downhill from there however. Magic Users start to get higher level spells, and their effectiveness continues to increase at a fast pace because of that. Level 2 has a number of really potent spell options. If you even get one decent level 2 spell, you will be getting ANOTHER doubling of effectiveness basically at 3rd level. It should be worth something like 8000XP, but the actual number is a mere 5000XP. Clerics at 3rd gain up to THREE level 2 spells, again this is a really large effectiveness increase, close to 100% better (remember, PHB says you have two 15s). The fighter OTOH is going downhill in terms of gain, he gets another 50% hit point increase, a +1 to-hit, and by now he's probably managed to get plate, so his XP for 3rd really should be on the order of 3500 or so, under HALF that of a wizard. Pity the thief, 3rd level means squat to him. The hit points will be nice, but he still cannot improve his AC any, and his abilities all still suck. To say he should require 2500 for 3rd level is almost harsh, but we do see that he's actually not far out of line.
It goes on like this, except the melee classes get harshly punished at higher levels, and the cleric never really does come into line at all, and the wizard gets a huge windfall. Some of it almost seems to work, like non-casters are not far off, but spell casting is just drastically discounted.