Cambridge and Oxford (as cities) aren't that different in size, as far as I'm aware, but Cambridge is distributed over a wider area (Oxford is a bit more compact as I understand). The universities have similar numbers of undergrads overall (not sure how specifically the law cohort sizes compare).
They have very nearly the same range of papers, except Cambridge has some half papers and seminar courses offered that vary year on year. The major differences in terms of subjects studied is that at Oxford more are compulsory, whereas at Cambridge you can choose between them more. Presuming you want a qualifying law degree though, the only realistic difference is that at Oxford they are required to take jurisprudence, while at Cambridge it's optional. I believe the number of optional subjects is the same when you account for the jurisprudence difference.
A more significant difference is that at Cambridge they sit exams at the end of each academic year, while at Oxford the exams are done in two lots; the end of first year, and then the end of the final year. This means they sit exams for all courses taken in the last two years of the course at the same time. Also at Oxford they wear sub-fusc for exams, apparently. On the one hand this means you have a very intense exam season at the end of your degree, on the other hand you might find second year a little less intensive, which may be more amenable to looking for vacation schemes etc.