It is all physics in the end .
A 7 foot longer trailer is a longer LEVER .
The length behind the trailer axles is uncontrolled weight which is trying to throw you off the road .
By having a heavier ROCK ( tow vehicle ) the LEVER has to work harder .
By having a higher percentage of length between the TT axle and the hitch you have more controlled weight and mitigate some of the Leverage behind the TT axle .
Having a Heavy payload Tow Vehicle one can concentrate more controlled weight onto the hitch.
Having a Lighter payload Tow Vehicle restricts the amount of tongue weight allowed to be concentrated on the tow vehicle .
A 9200 # GVW tow vehicle towing an 8200 # trailer is better than an 8200 # GVW tow vehicle towing a 9200 # trailer .
The further aft the trailer axles are in relation to the total length of the trailer the better handling the setup will be .
This is shown dramatically when towing boat trailers which have their axles placed considerably more aft because most current design boats have their engine weight concentrated at the stern .
The best trailer towperson can be helped by good choices in a heavier payload TV pulling a shorter / lighter TT .
Especially if he is taking a nap while letting his kid / wife spell him on a long trip for an hour or so .