The US wants increased spending because many small European countries spend that money in the US.

Honestly, the interesting thing question here is: (A) Does the EU have to political will to bankroll Ukraine. (B) Will the US keep selling weapons the Ukraine, if funded by the EU.

I suspect (B) is a hard YES. Anything would end US military industry.

On (A) I'm less certain, the EU has had a hard time finding consensus. The EU can do big things, even when it's hard, the EU did so under covid.