Yes, there is a significant size mismatch in this game and, no, these teams are not mirror images of one another Their offensive styles are very similar, but defenses are very different.
LSU's d-line averages 6' 4.5" and 273 lbs. Bama's d-line averages 6' 2.67" and 305 lbs. LSU's LBers average 6'1" and 218 lbs while Bama's LBers average 6'3" and 253 lbs. Lastly, LSU's defensive backfield averages 5"11.75" and 188 lbs, while Bama's DBs average 6'1.5" and 204.5 lbs.
ESPN and CBS may not be talking about Alabama's defensive backfield, but NFL scouts are. Dre Kirkpatrick is rated as the #1 CB in the nation, Mark Barron is rated the #1 SS in the nation, and Robert Lester is the #1 FS in the nation. LSU has a great secondary, and no one can doubt the playmaking ability of Tyrann Mathieu, but LSU's secondary does not matchup with Alabama's seconday in size, ability or coaching.
Yes, both teams have a powerful running games. The difference is one team's defense is built to stop a power running game, and one is not. Alabama's road-graders on the offensive line will netralize LSU's speed by running right at, through and over them. Alabama will do the same thing to LSU's defense as they did to Florida. LSU's defense simply does not have enough lead in their ass to stop it the vicious ground and pound they will experience in Tuscaloosa.
The outcome of this game will not be in doubt in the 4th quarter.