In the legacy Block Ack Mechanism, the bitmap size was 128 bytes. This was due to the fact that fragmentation was allowed in 802.11a/b/g. A block Ack frame could ack 64 complete MPDUs and each MPDU could in theory be broken into 16 fragments. Hence, a packet could comprise of 2 octets (bytes) in the older block Ack Scheme. Since, 64 MPDUs can be Acked and 2 octets per MPDU made the bitmap to be 128 octets in size.

In 802.11n and 802.11ac, fragmentation of packets was not allowed. Hence, each bit in the block Ack bitmap corresponded to one complete MPDU frame. Hence, the block Ack Bitmap was reduced to 8 bytes from 128 bytes.

The compressed block Ack bitmap is shown below in a wireshark capture

Partial State Information and Full state Information for Compressed Block Ack