Different producer, and the bass followed the guitar a lot more than it did on many of their previous albums. Those were two of the most mainstream albums Megadeth ever did, unless you count Risk as a Megadeth album, and the songs were much simpler (for Megadeth at least). You know, the guitars used/pickups and prefered distortion make a difference as well. Yes, I do agree that Metallica always had more of a crunch, but that doesn't mean they were always better produced. They also had the bass literally riding the guitar 90% of the time, so of course the guitars are going to sound fuller than what they really were.
If I'm coming off like Megadeth never had shitty production on an album, or Metallica did not average a better mix, that's really not what I mean at all. I just do not believe Metallica had some superior tech that Dave and co just could not understand or that they never came close to or exceeded. MoP was the best produced album prior to the Black album, but those that came before and after were only above average as far as what you would hear in the 80's. And Justice was fucking weak mix wise in comparison. Killer album, crappy production. The weakest Megadeth albums were Killing and So Far So Good. If you have ever read their history though, drugs and firing producers halfway through production probably played a significant part in that. Budget is also a factor and when you're spending half of it on drugs, your final product isn't going to be what it could have been.