It turns out that center Dominic Raiola isn't the only Detroit Lions offensive lineman who added some weight this offseason. Lions offensive tackle Riley Reiff also bulked up this offseason, according to Tim Twentyman. Reiff apparently played at 305 pounds last year during his rookie season, and he's now up to 315 pounds after gaining "10 pounds of muscle."
Reiff hasn't discussed the specifics behind his decision to get bigger, but it's a safe assumption that it involves him being the starting left tackle. Reiff didn't necessarily need to be bigger than 305 pounds last season given that he served mainly as an H-back/sixth offensive lineman, but this year, with him set to start at left tackle, perhaps the extra weight will come in handy.
With Reiff adding 10 pounds, the Lions' projected starting O-line looks like this from a size standpoint:
Last year's starters at left tackle, right guard and right tackle were Jeff Backus (6-5, 305 pounds), Stephen Peterman (6-4, 323 pounds) and Gosder Cherilus (6-7, 325 pounds). This means the Lions have gotten slightly bigger at left tackle and right guard, at least from a weight perspective. At right tackle, Cherilus is bigger than both Fox and Hilliard, so that's one spot where the Lions have gotten a bit smaller.
UPDATE: Fox is reportedly up to 318 pounds.