I disagree that it's all about size. I think it's a bit more complex than this. If this were true, then why would, for instance, some small branches of Christianity be considered "cults" and others not? The Quakers are usually not considered a cult. The Branch Davidians are.

Many people think "cult" when they hear an amount of "strangeness" in the "cultists" beliefs. Now, the first thought is, obviously, that "strangeness" is arbitrary, right? Christianity, when viewed from the outside in, is just as strange as Hindu or Islam or whatever, right?

Well, I think not, actually.

If you think of religions as memes, in the Dawkins sense (in The Selfish Gene, back in the 60's, Dawkins coined "meme" to explain ideas that experience natural selection but in a "meme" pool instead of a "gene" pool), then they also have memetic properties that either enhance its ability to survive in the meme pool, or detract from its ability to survive. Christianity is made of many memes (a "memeplex", as Dawkins said), many of which work great together. For instance, the "faith" meme that holds belief without evidence is heavily reinforced with the "hell" meme that provides a punishment for thinking "dangerous" thoughts. The "hell" meme also works great with the "evangelism" meme, causing it to spread quickly as this takes advantage of natural human instincts to try to help other people. These all form into a meme that survives great in the wild.

Cults, on the other hand, include memes that seem "stranger" than religions because they include memes that stand out as obviously destructive to itself. For instance, doomsday prophecies are obviously timebombs for the meme which will take it out of the memepool on a specific day. Ritual suicide is another self destructive meme.

Often, organizations which identify cults and help cultists escape from cults will have criteria to identify "cults". If you look at many of these lists, they'll be things like "encourages members to cut off from society and family", "requires members to give away all possessions", etc. There are, I think, common threads found in "cults".

Now, I would agree that the overall nature of "religion memeplexes" are very related to the nature of "cult memeplexes", but I don't think it's as simple as "cults are just tiny religions". Rather, I think that these are two emergent systems within the larger system of human society, and that they both have distinguishing traits.

I think it is common, though, that cults grow into religions; however, I think the history of religion clearly shows that when this happens, the cult sheds many of its cult memes. For instance, Mormonism dropped polygamy and isolationism. I suspect that Scientology is currently dropping many cultish-seeming traits in order to grow. Back when it was a cult, proto-Christians believed that the world would end within their lifetime, and the Bible even says this, but as it became a religion, their imminent doomsday prophecy became a metaphorical, nebulous, warning about some unknown time in the future, and transformed into a potent motivational meme for a larger religion. Perhaps this is a pattern because cultish memes are great for forming small devoted and exclusive groups, but don't work with larger groups, while religion memes are great for organizing huge groups of diverse and inclusive followers, but don't have enough zealous ideals to spontaneously start up.