there is a difference between:
25% of the UN budget comes from the States
and
25% of the budget of the States goes to the UN
If it's the first one, it's pretty normal... big country, lots of active working people, good economy,... they can AFFORD the support. Countries like Swaziland and Lesotho simply can't give that much
the second one is of course bullshit cause no one will give that much