Case capacity help show the difference. A 28 Nosler holds approximately 93 gr of water. The 284 win holds approx. 63 gr of water.
This is just over a 32% increase in capacity. This increase will yield an approximate increase in velocity by 1/4th the gain in capacity. So in theory the 28 Nosler will have a 8% velocity gain over the 284 with all things equal. I didn't get into seating depth which could alter this percentage.
The short SAMMI spec of a 2.8" OAL for the 284 may cause longer bullets to be deep into the case changing this relationship. If the 284 round has the bullet seated out to a longer OAL it might be as I suggested above.
These cases have different head diameters, body dimensions and lengths. The 284 win is 2.180" long and the 28 Nosler is 2.590" long. ETC ETC.......