I bought the 25th Anniversary "Goodfellas" Blu ray from a reputable dealer on eBay. It looked good from the bottom part as it was shiny, but I checked the amount of GBs and it said it was around 27.8 GB on the disc. Is that the amount it's supposed to be? Did I get ripped off?

I starting to see a pattern here...

Sounds possible, Warner usually aimed their encodes to take up as little space as possible. They'd often leave BD-50s nearly half empty for major theatrical releases.

I seem to remember many early Warner Blu-rays were created from the same encodes as those that were created for the HD-DVD format which maxed-out at 30GB.

According to DVDBeaver, the official Goodfellas Blu-ray is 24,474,154,580 bytes in size. It also uses VC-1 for the video encode, which is very rare for a Blu-ray, but very common for HD-DVD. So orangerunner is most likely right, they just took the same encode from the HD-DVD and put in on a Blu-ray.

Also, the "20th Anniversary" release looks to use the exact same disc as the original Blu-ray release.The size is the same, encode the same, same extras, hell, the bitrate graph is the same.
Goodfellas Blu-ray Robert Di Nero