Ryan Anderson is technically the Sam backer but he's on the build quite a bit in nickel...but with his hand in the dirt. Evans isn't smaller than Tim Williams...lets be real.
maybe Anderson is the starting SAM, but he's also the starting JLB then - regardless - he's the #1 OLB that has the biggest role in the most packages. A. Jennings is the starting Sam for the few times we run the 3-4, and in goal line packages, with Anderson being the other OLB. Point being most of the time, we play with only one OLB on the field at a time. Williams is a backup in both the base 3-4 and the nickel groupings and his starting role is in the dime rush package with Anderson being the other guy on the edge.
You're thinking the old Williams of his early years. One reason he came back was to add more bulk and prove he could still be explosive with the extra weight. The old Williams wasn't considered to be a 3 down player. Williams is now 250+ and is able to play in rotation very well in the nickel package. Evans is only 230. Huge difference. They're not even close to the same size. Evans is much faster however.
As you mentioned, in our most common package, the nickel, one OLB plays with his hand in the dirt much of the time like a DE. The other OLB comes off the field and is replaced by the star DB. Pretty often the OLB will take a two point stance, but still be right on the LOS. On rare occasions he will play off the LOS. When he is on the LOS, in either the 2 point stance or with his hand in the dirt, job #1 is to set the edge. He's got to take on either the OT or TE and hold the edge at the point of attack and maintain outside leverage to force everything back inside. To get that job done takes leverage, power and a decent amount of size. Speed and burst don't really create an advantage. Job #2 is to rush the passer - speed and burst are required here of course, but those don't matter unless you can do job #1. Evans is great, elite really, at #2 due to his speed, but right now doesn't have the mass/power to get #1 done.
so again, unless we somehow change schematically in a way that changes the responsibility of the OLB playing up on the LOS, I don't see Evans being successful at OLB in either the 3-4 or nickel packages unless he can somehow add 15-20 pounds in the offseason.