FYI, spanish post do also have size restrictions, but they only apply if you want to send something; they will reject it at the post office. I have had exhaust sent from Germany and the spanish post delivered to my door.
Seems that the delivery companies have agreements with foreign companies -i.e., spanish post will deliver german post, but another company will deliver german GLS parcels etc. As the foreign company may have different restrictions, they find agreements based more on economics than on keeping with the other company's regulations. So they make exceptions in those cases.
