I'm assuming that when you say "realistic fiction", what you really mean is "non-fantasy fiction". Then the answer is, of course fantasy has less of a readership than "realistic" fiction, because you're comparing one genre against the collective power of all other genres.
If we start looking at comparisons between genres, fantasy is probably at the low-end of the scale. The statistics on genre sales I could find (for US sales, 2009) suggest romance is king, with around 21% of book sales. Science Fiction and Fantasy together account for approximately 7.8% of the market, with Mystery taking up around 6.7%. I imagine if you split Science Fiction and Fantasy into separate groups, Mystery would then be higher than both. Keep in mind, that's just raw sales. If you start looking at core readership numbers, I suspect speculative fiction will be far, far lower.
Despite its small readership size compared to other genres, speculative fiction readership more than makes up for this by the fact that they hoover up just about anything good that they can lay their hands on. Orson Scott Card has written about this before in his book "How To Write Science Fiction and Fantasy", and goes to great lengths to point out that speculative fiction lovers reward good authors in spades. They're also more often open to the strange and the wonderful that just doesn't "fit" elsewhere. In short, there's a lot of opportunity for writers in the fantasy genre to have a captive audience.
Which leads on to your question about whether "a fantasy book will garner much less interest than a realistic fiction novel". You need to define what you mean by "interest". Do you mean raw book sales, with huge success across the world? Massive publicity and fanfare? A book that is read by anyone and everyone, despite the fact that it's fantasy? Look at the Harry Potter books as a demonstration of just how much interest fantasy books can generate. Or Neil Gaiman's work. Or Terry Pratchett. Or the Twilight saga.
Okay, you may say that those are just exceptions. Sure, not all fantasy books have such great success. But that comes back to what you mean by interest. Do you mean interest within the genre? A well-written fantasy book may not always be on the top best-seller lists, but it will generate a heck of a lot of interest among fantasy lovers. Consider the Malazan Book of the Fallen series, one of the best fantasy works I've ever read. Didn't make big waves outside the fantasy genre, but it sure as hell generated a lot of interest.
The long and the short of it is that you will always have a market that is interested if you write good fantasy fiction. Don't worry if that readership has smaller numbers. Write well, write what you love, and you'll be rewarded.