John Gill Commentary 1 Samuel 17:38

"And Saul clad David with his apparel, and he put a helmet of brass upon his head, and he clad him with a coat of mail." — 1 Samuel 17:38 (ASV)

And Saul armed David with his armour
Not with what he wore himself; for it cannot be thought he would strip himself of his armour in the field of battle, and when just going to it; and besides what suited the one would not be fit for the other, their bulk and stature being different; but this was some armour Saul had brought with him, besides what he himself wore, to furnish any with that might want it:

and he put an helmet of brass upon his head ;
such an one, though not so large as Goliath had, these being usually made of brass; (See Gill on 1 Samuel 17:5);

also he armed him with a coat of mail ;
which probably was of brass also, and like that of Goliath's too, only lesser, (1 Samuel 17:5) .