As wished by the original poster, the comment as an answer:
It is very likely that Google Docs does calculate in the size of the rendered document (kind of plausible; they render the PDF and make it available as Google Docs document). In this case, you may easily get 1 MB per page, no matter how small or big the PDF is. And if you deal with badly prepared press-ready PDFs, you very easily can fill up your hourly quota.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…