Not necessarily.
For example, in Langrisser III and DE, there are a couple of pretty big dat files. In III's case, a 12 meg one, and Langrisser DE's, a 4 meg. What are those huge files used for you might ask? Text. Graphics possibly as well. Amazing how wasteful NCS got when they decided to make games for saturn. Though for the most part, it seems alot of companies just use dat files for some kind of data compilation. Whether it be text, graphics, sound, etc.
My best suggestion is just to go through the file with a good hex editor and see if you can find any headers or possibly any kind of pointer tables at the begining of the file. If you find a table, see if you can trace where the various entries are pointing to, and you might be able to figure out what kind of data you're dealing with.
Cyber Warrior X