I apologize for the lack of real details, but I recall reading something about this subject recently, but I am having trouble recalling the specifics.  From what I remember, there are multiple (two?) buffers and counters associated with determining how big of a packet will be sent.  The purpose was to avoid network congestion and there are a couple of different "named" algorithms that govern the packet size behavior to this effect.  The net result is that even if the receiver can accept a larger packet, there are several conditions under which smaller ones will be transmitted.  You may want to try searching for TCP/IP congestion prevention or a similar set of keywords for more information.