HVAC sizing is a wild guess
When trying to determine the correct size for heating or air conditioning, what matters the most is
- the quality of the insulation in the walls and ceiling
- the "solar gain" -- heat being absorbed from direct sunlight, which adds 1000 watts (3400 BTU) per square meter (100 W/square foot) of radiant heat square-on to the sun.
However, these things are difficult to model, and require actual mathematics. Who got time for dat?
So air conditioning people go with a simpler number: Floor area and sometimes ceiling height, just to obfuscate the matter. That's ridiculous. That has no bearing whatsoever on HVAC load. You can have two houses with identical floor area/height, but one is direct sun with no insulation, and the other is LEED 5, all that stuff, in shaded forest. They will have wildly different HVAC loads.
You know better than them
Because they are guessing, your direct experience with the unit is actually more accurate than they are. You would remember sweating through half the summer while the unit ran continuously, or having to use 4 blankets on winter nights while it ran continuously. If that's not happening maybe the unit is OK.
So you tell me. Is the unit you have now satisfactory to heat/cool this space?
Too big is bad? Maybe on old units.
I do not agree with the HVAC industry's obsession with "right-sizing" - partly because they can't estimate accurately (see above). The main reason they don't want to oversize is to sell you the smallest unit possible, so their bid price is lower, so they win the bid!
There is a traditional problem with a too-large unit, however that applies to the older "one speed, on/off" units. That is if the unit achieves its goal too quickly, less dehumidification happens. So you might need to watch your relative humidity inside the room and back it up with a dehumidifier if needed.
However that should not be a problem for an inverter heat pump as you say you have. The inverter drive means it can run at variable speed to suit conditions, and will run on "low" for an extended period, assuring that dehumidification happens normally.