Big compared to what?
I dunno, I feel there's something kind of wrong with using jungle and 2step (or even broken beat) as a benchmark for grime's likely worldwide success. Those are all dance music forms (and fwiw, are dance music forms that as a percentage of the total dance market might well be as big globally as they are in the UK, certainly true in the case of jungle). And non hip hop dance music, UK or not, has not been big in the US since disco.
Whereas grime, if it hits at all in the US, will hit as a variant on hip hop, which makes a comparison with, say, the Beatles and the Rolling Stones almost more apt - it's a UK interpretation of the dominant US popular music culture, a culture that was originally black and urban in origin, but is now much bigger and broader than that, and which is not confined to clubs but lives equally comfortably on radio or in bedrooms.
Grime is intrinsically no more different to US mainstream hip hop than dancehall or reggaeton are. I don't know much about the history of it, but I bet the Beatles were originally popular with US hipsters, and I bet rock in the 60s was about as 'black' in terms of the demographic buying the records as hip hop is today.
That's not to say that any grime artist currently on the radar has Beatles-style hugeness potential, but if there's any worth at all in historical analogies (and there may well not be) then it's got to be considered at least <i>possible</i>...
Just a thought, anyway.