Have you actually turned in your exploration data to Universal Cartographics at your nearest station? Sounds to me like you should be getting more than that (I know I do).E.g. I have just found an unexplored system with 32 objects (mostly a mixture of ice moons/planets with a few metal rich and a water world) I spent several hours exploring every object and how many arcs did I get for my efforts... The grand total of 3
Anyway, the big ARX for exploration (I'm not counting exobiology here, but only cartopgraphics) happen if you scan lots and lots of systems, and especially if you get "first to discover" and "first to scan" accolades (although those are not necessary to get a good payout).
(Of course if you really want to max out your weekly ARX bonus, you would do exobiology.)