I'd probably be just fine with the 10k if it was just for a few times a year to tow back and forth from a house.
As for a heavier load trailer being bigger- that should make no difference. The next step up I-beam trailers go with larger I-beams, not a longer trailer. If someone is informing you that the next size up will not fit the boat then they are trying to sell you something, not trying to actually fit your needs. The cost difference between 10,12, and 15k is very minimal.
Here's a pic of my 15k trailer. Notice the I-beams actually are shorter than the boat, and the bunks extend to the end of the hull perfectly. There is just the right amount of room up front to extend past the bow pulpit, and I would not want any shorter of a tongue.
Total length of boat on trailer from tip of the tongue to back of the engines in trailering position is 37 feet. In down position I can get just a hair over 35 feet.
As far as the weight goes....Please excuse my reluctance to accept those weights as realistic, but my real world experience simply does not match up. Items like rocket launchers, outriggers, radar, anchor and rode, ropes and rigging, tackle boxes, tool boxes with tools, extra filters, life raft, spotlight, safety items, etc. add up quick. Throw in a couple of SCUBA tanks, a cooler with ice, chairs, etc. and the weight quickly climbs.
I agree that you can unload the boat and the 10k (carrying weight not GVWR) will be acceptable. Be sure to check and MAKE SURE that the 10k is the actual carrying capacity of the trailer, not the GVWR. The Ibeam trailer will weigh about 1700 lbs or so and some trailer manuf will make their trailers sound better by listing the GVWR which includes the weight of the trailer. A 10K GVWR trailer only has a capacity of 8300lbs.