I mean, more things enabled on your own particular firmware build.
There are plenty of features you can choose from. You can look at the config files of the m-firmware or the makefile of the k-firmware for grasping how many (similar or different) features each one has to offer.
Every function uses memory, some more, some less. A larger font user more flash, and a color screen requires more code than a monochrome screen (and deppends on the particular controller), etc.
Let's take one example: to show the E12 or E24 or E96 standard values next to a resistor measurements, so you can say whether the particular piece you're measuring is within range or not. Of course E96 with colour codes will use more memory than E12 only text. You can also choose to show standard values for capacitors.
Another example: there is an option to monitor DUT placed between Terminals 1 and 3. You can monitor L, or C, or R, or R and L, or R and C, or R, L, and C. The memory requirements are different.
So maybe somebody wants to enable every single feature the m-firmware or k-firmware has. I don't whether 64KB of flash will suffice for that, neither I know whether the RAM would it be enough too. Also keep in mind some features requires extra hardware.
Bottomline: me and others look at the source code, check which things like better than others for our job/hobby, make the file, check whether fits our unit or not, until we find the best tailored binaries for our likings.