I wouldn't worry about trying to compare Fotolia with other agencies' acceptance criteria.  They reject most of my stuff and I simply don't care anymore.  The more I upload the less I make. They are a failing micro stock company and I hope they go out of business.

I'm not saying, I get 100% acceptance rate everywhere else, but for example if shutterstock rejects something, I really get their reasons most of the time.
Fotolia cuts about half of my images and that's very unusual, especially since some of the rejected ones belong to my best sellers. It feels very aribitrary.

I don't see it that negatively if an agency doesn't reject that much. Sure, they should filter a little what's coming in, but why reject things where you can't tell in advance whenever they will sell or not? One can be easily surprised what people want. My personal experience with fotolia reviewers is, that they are not very skilled fortune tellers But I can always be wrong. Maybe their customers are so different that those images really wouldn't sell. However I find that hard to believe.

A good acceptance rate is friendly and motivating to the contributor and gives him a chance to expose all his ideas to the market. I discovered 3 niches in just a month that way.  Second of all, if those images are so bad, they will never sell or appear on the first page anyway (well, eventually in the "new"-section, but they will disappear from there pretty quickly again). And about the storage, I wouldn't mind if they send me an email once a year, asking me to confirm, which images of the unsold ones I want to keep online. Maybe even with some automated analysis giving me clues why they don't convert. I'm perfectly happy with deleting stuff that keeps my customers from seeing what else I have.

I'm just saying, instead of doing so much fortune telling, at the end the customer himself should decide whenever images are worth something or not. That's the only objective way of determining the true value of something. Reviewers should only be there to reject obvious and common sense cases. Having 1000 images of the same thing IS such an obvious case.

As a customer I would rather prefer a tighter selection

As a customer I wouldn't care unless I begin noticing that my search results are spammed with low quality images. And a broader selection makes it more likely to find what I look for, doesn't it?

Well, the number of images an agency has is strategically important, whenever one likes it or not. Working hard is worthless if you don't work smart. If you look it up in literature, this is called taking advantage of "positive network externalities". As long as the quality standard doesn't fall too low, quantity is very important factor when they want to rule the market. It's not just psychological.

In that sense, those agencies where you get >99% acceptance rate are not that stupid at all. As long as they don't accept eye catching crap (in which case they are stupid), they still have a chance on the market. Especially if their competition makes a mistake.