In the 70's and early 80's, there was a lot of tinkering with the injured reserve rules.
I believe that at one point in the early to mid 70's, teams could put players on the injured list and then take them off the list later in the season. Because of this, teams could use the injured list to stash players. Team rosters were smaller then, I think 43 with all players active on game day.
To combat the stashing of players, the NFL changed the rules. I think they tinkered with the size of the injured list but eventually settled on making it so that a player put on IR was ineligible to return during the season.
Then, there was a big todo after the 1978 regular season when Billy "White Shoes" Johnson had been put on IR and was ineligible to return for the playoffs. I'm not sure if that directly resulted in rule changes the following year, but it certainly motivated the league to find some compromise between the two extremes.
The active/inactive roster thing didn't change roster sizes, it just tacked on several players to the roster so that a team could still field a competitive team while carrying injured players on the roster.
I'm pretty fuzzy on the exact details - I was just a kid, but I think the general ideas I'm presenting are correct.