People have under-line and over-line functions generally backwards.
Heavier lines are to have more control in close, giving you higher grain weight in shorter line length.
If they improve your distance, it's because you better feel the heavier line load the rod to improve your timing.
Lighter lines speed up the rod and get you max distance, often sacrificing the ability to cast close.
(this is the whole reason Loomis began the modern trend to under-rate rods, so they would appear faster in Ted Leeson reviews in FR&R)
A buddy from PNW found a 5' Harnell 604 from the early '60s and sent it to me to evaluate - here begins the whole blow-by-blow evaluation comparing similar rods, and using different lines. https://fiberglassflyrodders.com/forum/ ... 52#p378113
Also took it to Cibolo Creek, though flow was too low for fishing to be good, but it was the best nearby water to try the short fly rod in its best-use element. https://fiberglassflyrodders.com/forum/ ... 2&start=25