It's because of how the blockchain works. The pools pay out as soon as your balance there is 0.5 MMC. All these payouts come in on a different block. When you try to send 1500 MMC at once the transaction would have to include data from 3000 blocks.
The transaction is not too large by coins, but by data size. That's also the reason for the high tx fee for your send of 500 - the fee is determined by how many kB of data the transaction needs.