Is it a small thing? - Is it not enough for you (Num 16:9)? The allusion to “David” is in order to contrast his trust in God with his degenerate descendant Ahaz’ distrust.
weary - try the patience of.
men - prophets. Isaiah as yet had given no outward proof that he was from God; but now God has offered a sign, which Ahaz publicly rejects. The sin is therefore now not merely against “men,” but openly against “God.” Isaiah’s manner therefore changes from mildness to bold reproof.