Grapes are generally 12-15# per gallon of wine. I can easily fit 5# of destemmed (not crushed) grapes in a 1 gallon ziploc.
IME with berries, I get roughly 1 gallon of juice from 8-10# of berries when I steam them. 5-6# of berries fit in a 1 g ziploc.
I can't help with citrus yields.
I do have a small, 7.5 cubic foot chest freezer for my fruit. Right now it's holding 135# of grapes, 18# of elderberries and several 5# bags of blackberries plus some misc. There is still a bit of room left.
Technically, steam juicing does dilute juice, but it is a minute amount - not enough for me to be concerned about it. Depending on what you plan to do with the juice, it may or may not matter to you. Steam juicers are designed and work well for extracting juice for drinking.