Why is the grip so small on this DUNLOP Fort racket?

Racquet grips come in different sizes, and this must be just a very small one. I also suspect from your description that this is a cheap model which happens to have the Fort or Laver name on it. Racquet companies make a lot of very cheap models for beginners. You can build up the grip by removing the leather and wrapping tape or another grip under it, or by putting extra grips over the present one.

I recommend buying the generic Tourna-Grip. It is by far the best selling over grip ever made and virtually every pro uses it. If a pro's racquet handle is light blue, it most probably is a Tourna-Grip. They are cheap and easy to apply and remove. In your case, start with one layer and see if that gives you the right fit, then add another layer if needed. Each layer is about a buck. They come in packs of 3 to 30 each ($3 to about $24 for the volume discount). I change mine about once a month (5-10 matches) depending on wear. Using them after they start really wearing down will cause blisters, so be careful. If 2 Tourna Grips don't work, you may want to remove the grip on your racquet altogether and replace it with a thicker leather replacement grip. Any tennis supplier or sporting goods store will do this for you for free if you buy it from them (and don't try to do it yourself as it could destroy your racquet handle). Good Luck.