Perhaps it is an arbitrary limit picked out of concern for weight and / or live energy in a place normally meant as living quarters. Too much of either on a balcony would be a hazard, especially if everyone with a balcony was doing it simultaneously in buildings not really built with either in mind.
800w sounds low to me, especially on 120v in the US, but the rules may have been in place for older less efficient / bulkier panels in mind.
Mind you higher powers just need a licensed electrician to sign off and usually also have them permanently wire it in as those setups won't need an AC side plug so the costs of essentially a generator hookup over a fixed tie-in isn't worth it, and they require notifying the local grid. Might be that beyond 10kWp the grid can tell you sorry-capacity-not-ready (but the grid will have to fix that soon, there are rules) but yeah.
800w sounds low to me, especially on 120v in the US, but the rules may have been in place for older less efficient / bulkier panels in mind.