I hadn't heard of Fuji YKL but a quick google turned up this comment in a flickr forum:
http://www.flickr.com/groups/ishootfilm/discuss/72157620478944457/"So, what was the worst film you've ever used in a reusable camera?
In my case it has to be a YKL-100 film (some Philippine market Fuji-based film), the film has very low-contrast, the colors were bland, and it was grainy for a 100 film. It's weird because, I used a Kodak 400 Ultramax an hour before that and that 400 film got precise color and very little grain in very similar lighting."
So perhaps your film is just very bad. The Superia should be fine though.
The other main factor is wrong exposure; if your negs look very thin and underexposed you're likely to see a lot more grainy images.
The pinhole size will have nothing to do with it. Film development is standard C41 so that shouldn't either. It may be that the lab are over sharpening the scan, perhaps because of the "soft" nature of pinhole photos. That will tend to highlight the grain. The might be able to switch off any sharpening for you, I don't know - it might all be automatic.