Why is the size of DVD files so small??

Small compared to what?

To high resolution images, or anything basically,I have several dvds and I put them on the pc and the file player opens them to their actual image size, and it is very small, 5 inches wide at the most, and it has very little definition, yet if you watch them on a tv, even a big tv, it looks fine, and with detail.

TVs have DSPs (digital signal processors) in them, for example, which upscale the image quality; mainly so SD content (such as DVD) can look better than it otherwise would on our newer HD TVs.

Also, TVs are FHD 1080p (unless the newest UHD 2160p, of course), whereas computer displays are frequently HIGHER resolution than FHD 1080p TVs -- many are 1440p currently (like Apple's Cinema/Thunderbolt Displays).

To high resolution images, or anything basically,I have several dvds and I put them on the pc and the file player opens them to their actual image size, and it is very small, 5 inches wide at the most, and it has very little definition, yet if you watch them on a tv, even a big tv, it looks fine, and with detail.

Moving picture looks sharper than still images, because our brain processes moving pictures that way, removing motion blur. Is this what you mean?

no, when you play a regular dvd on the computer the size of the image is very small. If you blow that size to the size of a tv it would look terrible and pixelated yet on the tv it doesn't look that way but rather well defined.

Sounds like you need new DVD player software. Obviously whatever DVD playback program you're using is either absolute crap or is displaying the image in some kind of preview/thumbnail mode.

have you played a dvd on the computer?

Plenty of times. Looks exactly the same as on a TV if you're using the right software. What program are you playing them on?