First of all, I haven't seen After Earth and calling me a layman in physics would already be an exaggeration, I guess. So the following answer might be a bit vague and oversimplified (and physicists might excuse me for saying anything embarassing), but maybe sufficient for a SciFi movie.
A graviton is a hypothetical (i.e. yet to be found/discarded) elementary particle that mediates (i.e. models/represents) the gravitional force. It would thus be reasonable to say that in this case the strength of the gravitational force between objects is equivalent to the amount or energy of the gravitons exchanged between them, thus
more gravitation = more gravitons. The movie thus ventures into a so called quantum theory of gravitation, a yet to be fleshed out/proved or discarded model of our world. And given the Sci-Fi context, it's not too far fetched that at the time of the movie gravitons are an established and proved elementary particle.
In addition to that, the mass is, as we all know, the charge of gravitation, i.e.
more mass -> more gravitation. When speaking of "mass expansion", which I would understand as a rise in mass, the movie probably draws a bit from relativity theory, which uses a bit more complex (or "vague") concept of mass instead of the classic
more stuff = more mass definition, allowing "mass" to effectively rise in certain conditions (fast travel, high energy conditions).
So the concept of gravitons and of changing, especially rising, mass is not something completely unknown to modern physics and it is not too far-fetched that both are somehow related, even if yet unclear in which way (or if at all) those theories could be combined in reality. I'd interpret that quote as meaning that a rise or otherwise measurable charge, i.e. a buildup or lineup, of gravitons has something to do with a rise in mass (although gravitation would actually be mediated by emitted and absorbed gravitons rather than actual particles "sitting somewhere in free space").
But I'm not sure if the exact causality of that quote (i.e. more gravitons lead to more mass) is completely reasonable, given that this direction seems less likely, as gravity increasing (by whatever means) doesn't need to lead to an increase in mass. But I also have absolutely no context of the scene where this is said, so maybe it was meant rather the other way around. He could mean that a rise in gravitons is an indication for a rise in mass, which doesn't sound too unlikely, given that
more mass -> more gravity and
more gravity = more gravitons.
So as a conclusion, even if it probably is practically gibberish and the screenwriters might not have put too much thought into it, I wouldn't downright discard it as completely ungrounded, especially compared to what other stuff is usually said in SciFi-movies and the fact that the movie neither has the goal nor the responsibility to explain every little detail in a 100% physically plausible way. But it may also be that After Earth's general critical impact (which, politely said, wasn't particularly high) or the fact that it maybe doesn't have a completely fleshed out SciFi-universe in the rest of the movie (don't know, though) might have influenced the judgement of those "many reviews".