One does not just "delete free space" in a NES game. Certainly you can't just "trim" it like maybe you could a GBA or DS game due to the console structure.
Not without significantly rewriting the game.
I think that comment warrants a wall of technical information, sorry if that is a can of worms.
It is more understandable if one knows how 8-bit cartridge games were structured.
8-bit CPUs almost if not always had a 64KB address limit, which meant that nearly all 8-bit consoles using those would be limited to 32KB ROMs.
Since 32KB would have been paltry even by 1985 standards, that is why they use mappers. (true of NES, GB, SMS, GG, MSX and so on.)
Mappers allowed games to have whatever size game ROM they wanted, as long as they had a mapper able to support it. Mappers break the ROM into "banks" so that at any one instant, only a total of 32KB of banks is visible to the CPU since that is all it can understand.
With NES, it is common mapper artchitecture that the LAST bank of PRG-ROM is always available to the CPU (to contain certain important code such as the boot code). If you remove data from the ROM to "delete free space", how will the game know where it is?
Also, official NES games often used CHR-ROM (where the PPU can access an entire ROM bank as the loaded tileset). That is, it's not just one binary blob, is a collection of ROM banks that are expected to all be an equal size each.
That means if you want to change even ONE TILE in the tileset, you need to duplicate an entire bank. (whatever the CHR-ROM bank size is, depending on the mapper/mapper configuration settings)
(one possible use was to cycle through CHR-ROM banks rapidly to create an animation effect, because using CHR-RAM and writing animation data to it every frame is a real bottleneck. See the USA (CHR-RAM) and PAL (CHR-ROM) versions of Dragon's Lair for what a dramatic difference that can make on performance.)
Though why the Chinese RPGs did it. Don't know if they made good technical use of it, but seeing as the ones spoken of are demakes like Pokemon and FF7, these were probably just slopped together like a bootleg-cranking machine.