I think that's true to a certain extent however prototypes exist for a reason. The reason tight ends are taller is that they matchup better in several ways that a fullback can't. First, they play on the edge, and consequently height and wingspan are important when matching up on edge defenders. Leverage in blocking is very important but wingspan in critical as well. Reaching the breastplate of a defender enables the blocker to effectively steer the other player. A lot of man on man engagement is locking the other guy out. The easier the defender can either lockout or keep the hands of the blocker off of him the easier his life is going to be. In order for a tight end to match up with these players he must be a taller guy.
The differences in the pass receiving game are at least four. First, tight ands and flexed out fullbacks are frequently jammed. This is usually done by the linebackers based off of personnel packages. A tall tight end has the length to disengage from a linebacker for a much cleaner release. A fullback will often be, at best, a draw in reach with most linebackers. This will hurt his ability to get a clean release. Once that player has released into coverage we speak about physical matchups. Tight ends are usually faster and usually bigger than a linebacker and always bigger than a safety. Not only can the tight end win body positioning stuff as well as creating a bigger catch radius, but his height ends itself to running seam routes and going up for the ball in the endzone. Fullbacks are a poor matchup as they lose on speed to safeties with similar heights, as well as on size and at times speed to linebackers. Even all world receiving fullbacks like Larry Centers are best used on leaks, flats, middle hitches, and screens. Fullbacks really struggle to compete in a game changing manner in the passing game.
While they can produce in the passing game it is limited to situational exploits and smart playcalling. The fullback is best existing as a run player who can account for extra run defenders, create seals, lead through to the second level, and provide the back with a clean read to explode off of. His biggest contribution is taking away one less player the ball carrier has to beat. If you have a good one, he should be able to get you a few tough power yards, catch the defense offguard on a quick handoff, and leak to the flat in the passing game. To expect a significant contribution with carries in the NFL is unrealistic. Triple option this is not. I know there is a lot of creativity that can be imagined with the fullback, but ultimately a lot of that is placing the player in a situation where he is poorly utilized. There's a reason Belichick has tried for the past decade to find Gronk before finding him instead of simply motioning around a fullback.
