Where'd my beer go?

I bottled the Czech pilsner that Joe and I brewed some 2 months ago, and was disappointed that there were only 3 gallons worth of bottles (12x22oz, 10x12oz). I can’t remember how much we racked to primary fermentation, but I’m sure we lost a bit when we dry hopped in secondary (with 2+ oz of loose Czech hops), and maybe 1/2 gallon from all the trub after lagering.

I’m really curious if a hop back can give the flavor without loosing the volume. There has to be a more efficient way. What would the Germans do? (WWGD?)