Redis master and slave use different memory

I deploy a redis replica set which consist of one master and one slave.

from top command output, I figure out each memory usage.

master: 8176352Byte * 8.5% = 678MB

slave: 2050116Byte * 75.6% = 1513MB

Why does slave use much more memory than master?

master top output:

...
KiB Mem:   8176352 total,  7746300 used,   430052 free,    55048 buffers
KiB Swap:  8385892 total,  2360512 used,  6025380 free.  1279644 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND     
32609 redis     20   0 1717424 696708    928 S   0.3  8.5 304:22.75 redis-server
...

slave top output:

...
KiB Mem:   2050116 total,  1977052 used,    73064 free,     3620 buffers
KiB Swap:  2096444 total,   259100 used,  1837344 free.   165552 cached Mem

  PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND      
 1675 redis     20   0 1725616 1.477g    672 S  0.0 75.6   0:23.03 redis-server
 ...