Sadly, the W5100 requires four byte on the SPI bus to transfer one data byte. Here is a quote from the datasheet.
In SPI Mode, W5100 operates in “unit of 32-bit stream”.
The unit of 32-bit stream is composed of 1 byte OP-Code Field, 2 bytes Address Field and 1 byte data Field.
There are two SPI op-codes, read and write.