SPI transceive read clarification: buffer size not corresponding to transmitted read bits

[AuFries]

I'm using spi_transceive_dt() to write a register read command then immediately read back 16 bits. However, despite having a receive buffer of 16, only 8 read bits are transmitted by the function. Why is this? 

transceive on a logic analyzer:

static uint8_t tx_buffer[1];
const struct spi_buf tx_buf = {
	.buf = tx_buffer,
	.len = sizeof(tx_buffer)};
const struct spi_buf_set tx = {
	.buffers = &tx_buf,
	.count = 1};
static uint8_t rx_buffer[2];
struct spi_buf rx_buf = {
	.buf = rx_buffer,
	.len = sizeof(rx_buffer),
};
const struct spi_buf_set rx = {
	.buffers = &rx_buf,
	.count = 1};
spi_transceive_dt(&cadc_spi_spec, &tx, &rx);