Here is the misconception, in college, many teams put their best athlete at LT regardless of their skill set because the edge is generally where the best pass rushers are in college. So, while 6'2 315 isn't ideal for a LT, in college if that guy is your best option you will play him at LT. See Senio Kelemente who played LT for Washington. See Barrett Jones in Alabama. This is why you see several teams drafting college OTs and moving them to OGs. Teams essentially will look and see the best player on that teams line, and if his skills translate to OG, then they draft him and move him. The most common reason for this move is height and arm length. Short, squatty OTs rarely work in the NFL.
So, why can't just any OT move to OG. There are a couple reasons. First is leverage. In order to play OG you need to be able to play with great leverage because the difference in size between OGs and DTs is not very great. This is why shorter OGs are normally better than taller OGs. Second is difference in movement between the two positions. OTs generally will drop set more often than OGs because they are normally more subject to an outside rush. OGs have to be better laterally than OTs because they have to be able to pass off rushers. OGs alse need to be much stronger because they have to be able to enguage a defender, disenguage the defender, slide and then enguage another. Long OTs generally aren't suited for this.
In terms of Bobby Massie and Levi Brown, here are the reasons why, IMO, they won't work as OGs. Neither play with great knee bend which hurts them in terms of playing with leverage. However, because they are bigger than most edge rushers, they can still have a strength advantage. Against bigger, stronger DTs, they generally will struggle because they play too tall. Also, neither are tremendous athletes which is why both have played their best football out of the RT position. With a very athletic OT, it is just an issue with strength when playing inside because they already have great feet. With a less athletic OT, it becomes both a strength issue and a footwork issue. So, in summary, a lack of top athleticism, footwork, and playing leverage are what keep Brown and Massie from being OGs options.
Essentially, this is what you are seeing with Daryn Colledge. A drafted OT by Green Bay who failed and was moved inside. He then became their worst offensive lineman and they got rid of him.