I assume you are using a flash-modded ipod?
The original firmware can't handle more than a certain number of tracks, because its database size is limited to what can fit in RAM. I forget what the number is, but you can' probably google it (maybe 10,000 tracks?). Nothing to do with rockbox 'stealing' anything, it's just the limitation of the original firmware. That's one of the main reasons for using Rockbox to start with.
As discussed here
https://www.iflash.xyz/store/iflash-compatibility/Depends what your ipod's original HD size was. The 30gb ones have only 32mbh RAM, the 60/80GB ones have 64mb, so can cope with a larger database. Also depends how much metadata your tracks have.