HTTP GET is designed so that all information necessary for the interaction is part of the URI, thus promoting URI addressability. With HTTP POST, some information intended to affect change to the resource state may be part of the protocol headers, not in the URI. With this approach, the resulting URI for identifying the resource may be shorter, but the advantages of URI addressability are lost.

As already mentioned by moritz the difference in size is unlikely to make much difference to bandwidth.

From W3Org