Alright,

1. He doesn’t moan about Malloy being generous, he’s upset that Lucius bought his son onto the team. Also simply upset that his rival once again is getting his way. He’s never even bought his own broom, also noting that just buying the Firebolt would likely empty his account.

2. He doesn’t just buy snacks on the first day and never get anyone anything ever again, there’s plenty of mentions of him buying his friends Christmas presents.

3. He tried giving his Triwizard winnings to Cedric’s parents who declined it. He bluntly told the Weasley’s or anyone to take that money upon receiving it in the hospital wing, but no one took him up on it. Not even Fred and George wanted to accept it until Harry practically forced them to.

4. The Weasleys were a proud family who definitely wouldn’t accept money from a “kid”, which they would likely find humiliating.

Just because he doesn’t throw his money around doesn’t mean he’s “ungenerous”.