I ran into a similar problem after I made some gear changes. There isn't any difference between the two when it comes to the volumes posted on their product pages.
Miniposa - 2,400 c.i. (39 l.) in main pack body/extension collar
Conduit - Main Body: 2,100 & Ext. Collar: 300
I found that most UL cottage companies GG, ULA, MLD, etc. don't offer much when it comes to smaller than 2,500 ci packs. Zpacks have an 1,800 ci pack, and Golite has a 25 liter pack. Black Diamond, REI, and Salomon have some 30ish liter packs made out of heavier materials and geared towards adventure racers.
I decided to substitute different sleeping pads according to how much volume is left over, which right now means deciding between a BA Insulated Air Core, GG Thinlight, and a generic closed cell foam pad. I really like the side compression chords that the Ohm has.
Anyone else have any tips for dealing with unused volume?