CS folks are not really used to upload their papers on the arXiv. So this is probably not a good indication of the number of papers published in each field.
Maybe not to the same extent as physics people, but there is still a lot of CS on arXiv. More so in some subfields than others, but there's a pretty steady stream of CS papers showing up there. Enough that one person can't keep up with reading and digesting all of them as they appear.
That said, I don't disagree that there's a lot more physics than CS on arXiv. :-) I'm just not sure if that's because CS people don't upload to arXiv, or because CS people publish fewer papers in general, or "other".