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What are the advantages/disadvantage of a 3/4 size horn?

Let's call Harvey Phillips old Conn a 3/4. Advantages include flexibility and ease of making beautiful sounds and relative ease of carting around.
Disadvantages: not the "resonance" of something like Jake's 6/4 York.

HOWEVER, Harvey could provide a solid "foundation" to a full orchestra.
Jake could produce gossamer fine sounds that could blend in any chamber ensemble.

Gary Offenloch once asked me, in Harvey's presence, which was the better player. My answer - both.

A bit like which was the better trumpet player Maurice Andre or Bud Herseth. Both said the other was. And I heard that when they were ace to face.
I knew, heard and studied with Jake and Harvey and my personal view is that when you listened to one or the other, it wasn't the horn that mattered, your preferences might matter to you but, either was very damned good.

It aint the chunk of metal that counts: It is the MUSIC that is put in, gets amplified, and heard that counts.

It comes down to finding a tool which you feel is the one "fits" so you make the best music you can: your "voice"

There is no replacement for displacement.

One answer to both: it's kinda small.

Depends on the acoustics of where you play. If your hall is highly bass-friendly then 3/4 will be fine...and if your hall is so bass-unfriendly that you are pssing into the breeze with any tuba, then 3/4 is also fine because you can at least thicken up the low brass sound even if you can't offer much fundamental. But for normal places, the normal size is normal for a reason.

I use a 621 for teaching and small ensemble playing, situations where I don't need a lot of volume or overtones. It works great in those settings, but it definitely lacks the resonance and "presence" of a bigger horn, which is why I always keep a 4/4 on me. I think a 3/4 is a great "garage" horn: nice to have when you need it, but not as the only tool in your arsenal