If I had to guess I'd say it is a combination of 1) the nature of 4e crunch, 2) the total number of game elements revealed quantitatively by the compendium, 3) some of the many interpretations of "everything is core" plus other variations of the completist urge, and 4) the fact that WotC is clearly concentrating on making every book have close to the maximum potential audience. Let me attempt to develop that picture, and you can judge whether it adds up.
First, whatever its merits or demerits, 4e crunch is pretty uniformly distributed across all classes and, taken as a whole, somewhat bulky. I don't necessarily mean individual pieces of crunch (a single power, feat, or monster is pretty darn compact) but supporting an entire class with new options (particularly powers) grows the page count pretty rapidly. All the books we've seen so far that provide new powers are also accompanied (to my knowledge) by a new build or something similar that demands adding many powers at once. In other words, we don't often get a "trickle of crunch."
Contrast this to 3.5, where spells (for example) were usually individually bulky but could be introduced piecemeal and only needed to support a fraction of the classes. The page counts between editions might not change much, but in 3.5 the amount of crunch applicable to a given player could change wildly depending on the types of classes they preferred to use. The net effect for 4e is that no class is as inherently compact as a 3.5 rogue or fighter, so players gravitating to those types are more likely to be aware of the growth of 4e options. I'd also guess that a disproportionately large percentage of 3.5 spellcasters (particularly wizards) were played by precisely the people that enjoyed the growth of crunch inherent in constantly introducing new spells, and would have greater tolerance of it. In other words, a self-selection effect. That is a very broad characterization, but it certainly fits the encyplopedic feel of the 3.5 wizard that some people think is missing from 4e.
The second point is pretty simple, the compendium has really made it clear how many game elements have already been introduced in the first year of the game. For most of us, I think, the "size" of the game is estimated from the crunch available to us. With the compendium we can be quite accurately aware of just how much stuff there is...and those numbers are pretty large. For example, I kept track of a decent percentage of 3.5 feats in a spreadsheet, and by the end had around 1600 from 34 books. At this moment the compendium reports 998 4e feats from all sources, and 729 from 8 rulebooks. Whether this greater pace in the number of feats is "too much" or makes building characters that much harder could be endlessly debated (generally 4e feats have simpler prereqs, simpler effects, characters gain more of them, and a great many apply only to a specific class or race) but the number feels huge.
Third, "everything is core" has been intrepreted by some people to mean that new crunch is "less optional" than in previous editions. Even if this is an unconscious response I think it tends to enhance the completist urge among consumers taken as a whole. The individual who resents these implications is probably more likely to think the amount of material is inappropriate, even if nothing else had changed from 3.5.
Fourth, and related to the last point, is the business model underlying 4e. Roughly, maximize the number of potential buyers for every book by reducing setting-specific material and emphasizing "generic" material whenever possible. Whether this is a good choice (financially or from the hobbyist's perspective) has been discussed pretty thoroughly in other threads, but it can enhance the "too much too quickly" perspective. In short, even if the total amount of material produced were precisely the same as in previous editions, every consumer that isn't a completist already is being marketed more material than before. From the perspective of the individual consumer that can feel like more material has been produced, and it potentially influences the perspectives of a large percentage of the total D&D audience.
These points aren't exhaustive, and they aren't immune to nit-picking, but I think the basic thrust is sound. Putting them together, it doesn't surprise me that the "too much too quickly" perspective is oft repeated.
I don't have a settled opinion on the topic, but I'd lean toward thinking the amount of material produced is reasonable given the structure of 4e. I like a lot about 4e, but my greatest reservation is the "sameness" of classes from a mechanical perspective -- that diversity was something I absolutely adored about 3.5 even if it caused the occasional problem. With respect to the current topic that is somewhat ironic, since even if 4e has a far greater number of possible options than 3.5 it generally feels to me like the possible differentiation is less.
