Specifying KEY_BLOCK_SIZE causes the heap tables to be created with Row_format = Fixed. The same CREATE TABLE but without KEY_BLOCK_SIZE results in a Dynamic table:

CREATE TABLE t2 ( f1 varchar(128) ) engine=memory row_format=dynamic KEY_BLOCK_SIZE=1024;

MySQL [test]> show table status like '%t2%'\G

• • • • • • • • • • • • • • • • • • • • • • • • • • • 1. row ***************************
                                                      Name: t2
                                                      Engine: MEMORY
                                                      Version: 10
                                                      Row_format: Fixed
                                                      Rows: 0
                                                      Avg_row_length: 130
                                                      Data_length: 0
                                                      Max_data_length: 1057472910
                                                      Index_length: 0
                                                      Data_free: 0
                                                      Auto_increment: NULL
                                                      Create_time: NULL
                                                      Update_time: NULL
                                                      Check_time: NULL
                                                      Collation: latin1_swedish_ci
                                                      Checksum: NULL
                                                      Create_options: row_format=DYNAMIC KEY_BLOCK_SIZE=1024
                                                      Comment: