Why the shortening of "fitted" to "fit"?
The definition of 'fit' used here is given by the OED (5.a.) as:
To be of the right measure or proper shape and size for; to be correctly shaped or adjusted to. Said esp. of dress; also fig.
One of the two earliest quotes for this definition in the past tense uses 'fit'; the other uses 'fitted'.
a1691 BOYLE Firmness Wks. 1744 I. 278 As much of the stone, as was contiguous to the marchasite..fitted the marchasite so close as if [etc.].
1795 BURNS Song, Last May, a braw wooer, And how her new shoon fit her auld schachl't feet.
So both forms have been in use for a long time.
Searching COCA, COHA, and the BNC for "fit well" vs. "fitted well," I found "fit well" outnumbering "fitted well" by 20:1 in COCA (indicating American usage), 8:3 in the COHA (indicating older American usage), and by 2:1 in the BNC (for you Brits). So either is fine, on both sides of the Atlantic, but "fit well" is much more common in modern American usage.
I think there are two separate verbs here that are the same in the present tense but different in the past.
- To fit the way clothing fits. "This outfit fits me fine. Did that outfit fit you? Yes it fit me."
- To fit as in to equip. "We fit our men with the best rifles. I was fitted with the best equipment."