With the double gusset folded up into the armpit, does it fold up to the point of being bothersome?
No. You do not even know its there. That is until you lift your arms.
Then the added room kicks in.
Most all Navy flight jackets incorporate this two piece type of gusset
in their sleeves.
It also helps the jacket from lifting up past your waist if you raise your hands
up over your head.
That usually results from a jacket that has its armpits too low, and thus,
when your raise your arms, it immediately starts to pull on the side seams
of the jacket, and makes the jacket raise up past your waist.
With the gussets in place, and with the armpits lowered for comfort, when
you raise your arms, the extra material in the gussets kicks in, and its
that material that moves, and it keeps from tugging on the side seams,
and making the jacket ride up your body.
Also, without gussets, if the armpits are made so that the side seams
of the jacket, and the sleeve seams come together closer together
under your armpit, then the jacket will not ride up your body as much
also.
The drawback to that is, if you have big biceps, or you have larger
arms in general, this can make the sleeves feel too tight under your
arms, almost to the point of discomfort.
Thats the basic reason for gussets.
Flathead