AFAIK the USB bulk transfers (512 bytes packets in High Speed mode) are used for actual data exchange. Any amount of data may be sent to OUT endpoint as USB subsystem on the host computer handles this automatically: device sends NAK on the OUT endpoint when its buffer gets full and the host computer reschedules the data delivery. Therefore there is no need to split write data in 512 byte chunks. And the data is buffered on the host computer. Same for the IN endpoint: host computer buffers data until either its size reaches the limit you've requested (e.g. 64 kB) or a timeout occurs.