I'm pretty new to poker and haven't really played many of the smaller field tournaments, mostly because they tend to have quick blind levels that I'm not used to.
However, I think if there
were an answer to this question, at least for someone who is grinding many tournaments per day and is seriously trying to earn money, it would be somewhere in the middle. In exactly the same way that one must increase the volume of tournaments played to compensate for the game's inherent variance, one should also diversify the types of tournament structures played during a session.
Regardless of whether you prefer large field or small field tournaments, it would certainly behoove you to mix in some of what you do not like to play for the benefits many have already mentioned.
The others have mentioned that large fields have high variance but high rewards, so mixing one or two of these into your daily session might allow for a big score every once in a while, and big scores are crucial to a climbing
bankroll.
On the other hand, others have mentioned that smaller field MTTs or SNGs have lower variance and lower rewards but offer more consistent returns on your investment. Therefore, mixing one or two (or more, since they are inherently faster) of these into your daily session can mitigate some up-and-down feel your bankroll may experience if your primarily play large field MTTs.