Most guides ignore the "digital litter" of the 3DS library. Every time you play a game like Animal Crossing: New Leaf or Super Smash Bros. for 3DS , the console downloaded updates. In a ROM set, updates are stored as separate .cia files (installation files).
Because the 3DS scene is mature, most sets have been verified with or 3dsconv . Always download from sources that provide SHA-1 hash files. A "complete" set with corrupted ROMs is worthless. 3ds Complete Rom Set Size
Is that reasonable? For a single console's library, yes. Compare it to the PlayStation 2 (~8TB) or Wii (~4.5TB). The 3DS sits comfortably in the "large but manageable" category. Thanks to the efficiency of the .cia format and trimming, the entire handheld library fits on a single external drive you can buy for $100. Most guides ignore the "digital litter" of the 3DS library