IMHO you made the right choice, you can make a big bag smaller but you can't make a small bag bigger as others have said above. You might find that the extra space provides opportunity to use the pack in less 'bushy' situations as well - my LA Cerro Torre was pressed into service for grocery hauling when my car was being fixed and I had to take the bus to the shops - made an annoying task slightly less so!
Also, I find 'milspec' is, in the main, a clever marketing buzzword primarily now used by certain brands to make it sound like a pack has some grandiose ultra-durable/life-saving/female-bedding capability not on offer with other packs. Whilst there are cheap, crappy packs available on the market, my twenty year old Karrimor civvy pack puts paid to the notion that something has to be designed to survive being crapped on by a tank to be useful. Here's hoping you enjoy your new purchase, be sure to update us on how you get on with it!
hahahahaha