"Jumbo" are just large Japanese quail. It's a marketing term more than anything. Some people will sell "Jumbo" quail that are average sized Japanese quail & others will sell their large Japanese quail without labelling them "Jumbo". They're all Coturnix coturnix japonica. Asking for average weight of their birds will give you a better idea of whether the birds are the size you want.
Larger quail are generally bred for meat purposes. Like more average sized Jap. quail it would depend on the individual line & their feed as to what size eggs they lay. If you're after particularly large quail eggs, ignore whether or not someone is selling "Jumbo" quail & just ask them what the average weight of their eggs are if you want them for that purpose. Ask them what they're feeding, too... as diet will play a part in optimising the size of the eggs that their quail lay (just like chooks and, I suspect, other birds).
Someone on BYP (my memory fails me as to who it was) worked out that their largest quail were hatching from the smallest eggs... whether that translates to other lines of Jap. quail, I do not know.
Commercial egg weights for Japanese quail in Australia tend to be listed as around 10 grams (NSW DPI factsheet).. from hens 120 to 160 grams ... I'd find that small from my backyard quail, and they're not what I'd describe as "Jumbo" either, nor do I make any great effort to breed for egg size (or overall bird weight). I should weigh them one day to see how they compare to the NSW DPI weight range. I consider 14 grams to be good sized / acceptable eggs from my quail hens as a fairly consistent figure, which is achievable on a decent feed. I have no idea what size eggs other backyarders get, so don't know if 14 grams is small for backyard quail or not.