They can be all bundled into a large archive file, though that is not so common.
It will also depend upon the type of game -- a 50 hour story driven RPG is going to be orders of magnitude larger than a puzzle game.
The type of text engine could also play into things -- many are not full markup but if I press ctrl+a on the previous page and save that in a text file it is some 40 kilobytes, the HTML file if I save the page (which does not count pictures or anything) is north of 200KB.
It is more that files measured in megabytes are more commonly graphics, sound, video, 3d models or something similar so if you are going through you can push those massive files to the back of the queue. Similarly anything that is only a few bytes is probably going to be an accessory file, little scrap of info that the game uses somewhere or something like that.
Size alone does not give me a full picture, when combined with file names, directory names, extensions, magic stamps (like extensions but the first few bytes of a file often indicate the file type) and then some further analysis that gives me things more quickly than I might if I have to go through each and every directory/file one at a time.
I also forgot to mention that the format Nintendo provides for text on the DS is BMG. Not that many games use it, even if they otherwise use the nftr font format, but it is worth knowing about.