In 2005 that was about what they could fit on the craft with weight, power, space limits. Now it just a single memory stick.
The 2003 Mars rover Spirit was among the first to use flash. Even though the OS was 25 years old battle tested version of UNIX called VxWorks, the flash driver was new. It had a bug that incorrectly marked the free inode list on flash disk. So about six weeks into the mission the disk appeared to be full and caused a safe mode reboot every few hours. The engineers successfully uploaded a patch and one of rovers is still operational.
And we're still not flying memory sticks (but we are flying more memory than ever before). Fun fact: New Horizons has no file system on its recorder, but current missions at APL now do.
"For data storage, New Horizons carries two low-power solid-state recorders (one backup) that can hold up to 8 gigabytes (64 gigabits) each." (From [1] linked above.)