I have heard of them being able to move larger aggregate, the rule-of thumb for a piston-type (high pressure) concrete pump is that the pipe/hose must be 3 times the nominal top size ofnte aggregate, but to be on the safe side, I subscribe to the theory of 4 times, so for 3/4" mix, it would take a 3" hose, but I used 4" slickline (the 'dead' system transport the concrete to the placing hose) myself, and a 3 1/3" x 40' long placing hose, that we would always move back when we broke back system. Any reducers should be as long as practical to reduce the pressure and possibility of plugging in the reducer. The reducer I had to go from 4" to 3 1/2" was 16" long and for 5" to 4" reducer, it was 36" long.
To pump a mix with 3/4" aggregate I would think you would need at least a 4" hose in the squeeze mechanism, a mix quite rich in cement powder and sand and likely minimal round rock as opposed to crushed. I don't think a mix that is normally pumpable in a piston-type pump would be pump in a squeezer, you'd need to cut back on the rock content and adjust the cement content upwards to account for the additional surface area imposed by the relaive increase in the mixes overall surface area.
Contrary to popular belief, you can get a strong mix out of round stone, although the stone needs to be consideraly cleaner in order for the cement to get a srong bond on the stone's smooth surface, as compared to the roughened surface of crushed rock.
There is a theory that you can actually get a stronger concrete with natural stone than crushed due to the fact that there hasn't been any stresses on the stones to cause micro-fractures within the interior of the stone to create the internal weaknesses that will fracture under the pressures imposed on the solid structure later. (this is the THEORY)
That being said, stone is actually a 'filler' in concrete. A single stone has much less surface area to be coated with cement paste than an equal volume of sand. You can get away with considerably less cement in a mix with larger stone and still acheive the same strength. the same is true for the amount of cement required to make the concrete 'pumpable'.