What Manon said except you can obviously outbid yourself, like this: 1,000>10,000>100,000>1,000,000>4,000,001.
Also what Cletus said, with the addition that some people overbid on Sunday before their funds reduce due to the Monday financial update, so they are able to bid that little extra bit.
Also, an autobid feature could have limited this, but was voted down by us, the users.