The 4-3 was designed to keep the o-line off the linebackers, so the linebackers can fill the holes and stop the run. 3-4 was designed as a run-stopping defense, freeing the linebackers to cover tight ends, etc. The 3-4 has to have a big guy in the middle since he'll get double teamed nearly every play, and he typically doesn't make plays, just does the proverbial clogging up the middle.
If you can find a 325 lb guy who can rush the passer, great, but those guys aren't easy to find, generally the bigger you are the slower you are, though of course there are exceptions.
Cowboys run a 4-3 most of the time, they need the quicker DTs, by design, the Rams do pretty well with a 280 or so pound Donald as the feature DT, but he has exceptional quickness AND strength. Charles Haley was a 255 lb DE, but stronger than the average 255 lb player. You would like to have the exceptional guy if you can find them, but those guys go early, Dallas hasn't really had, except for 2016, a really high draft pick, so to say they should draft the exceptional DT is fine, but usually they're gone by the time we draft...