Data is received from USB to the PC by a polling method. The driver will request a certain amount of data from the USB scheduler. This is done in multiples of 64 bytes. The 'bulk packet size' on USB is a maximum of 64 bytes. The host controller will read data from the device until either:
a) a packet shorter than 64 bytes is received or
b) the requested data length is reached
The device driver will request packet sizes between 64 Bytes and 4 Kbytes. The size of the packet will affect the performance and is dependent on the data rate. For very high speed, the largest packet size is needed. For 'real-time' applications that are transferring audio data at 115200 Baud for example, the smallest packet possible is desirable, otherwise the device will be holding up 4k of data at a time. This can give the effect of 'jerky' data transfer if the USB request size is too large and the data rate too low (relatively).