kAFL looks like not exploring enough payload size

I am using kAFL to fuzz HEVD (Extremely Vulnerable windows driver). Maybe i am doing something wrong but kAFL almost never generates payload of a size bigger than 256 (default kickstart is 256 so i guess this is because). In order to exploit some buffer overflow input buffer needs to be bigger, If i manually set the input buffer indeed it works but i do not think it is the right manner.

I am simply starting it with kafl fuzz -p 12 --log-crashes. And I modified the agent simply to use the HEVD driver and fuzzing every IOCTLs and printing the IOCTL when it crashes. (I also tried to fuzz only the IOCTL with buffer overflow but still payload size not enough to trigger the overflow)

I mean should i only increase kickstart size or eventually it will start to try bigger payloads?