Majora's Mask side-quests were there to get you to interact with the characters, which was Koizumi's goal with the game.
Besides, the game didn't have a long development time span. I think it was almost the same length as Wind Waker.
I think it was even shorter. Which makes the game even more amazing, to put such quality game in such a short time.
Majora's Mask is such a weird game, which is why I appreciate so much, I doubt we'll ever get a game like this, from Nintendo or any other developer.
I'd say it had MORE gameplay than either Wind Waker or Twilight Princess. Both of those felt long due to the vast amount of EMPTY SPACE you had to trek through, meaning there was a lot of time wasted for every step you took. In MM, there was plenty to do in so few seconds of travel no matter what direction you walked in. It also has the most efficient transportation system in the entire series, ensuring you didn't wait long before finding something worthwhile to do: standard teleports; standard worthless horse; high-speed Goron rolling on land; swift Zora swimming thru water instead of taking forever to get across any version of Lake Hylia; and the Bunny Hood which allowed you to generously move fast over standard surfaces while STILL retaining the conventional abilities of Link that no Horse, Boat, Wolf, nor Train can provide.
MM also maintained the equivalent of 8 LONG dungeons. 4 Primary dungeons plus the quests (which thoroughly tested your secondary abilities you acquired) required to gain entrance into each. Looking at a base duration of 3 hours (Slow Song of Time) for each dungeon/quest phase, that's at least 24 hours of gameplay by a skilled player -- the general concensus I gather is that MM's main dungeons were, on average, the longest and most difficult group of dungeons among the 3D Zeldas, so people are likely to spend more than 3 hours on it.
Ocarina of Time took me between 3 to 5 hours to beat a dungeon and clear all its treasures the first time through (it was 1998 launch period, I was 17, senior in high school, and this scope of 3D was still new to me and all of us). I beat MM's dungeon's within the 3 hour Song of Time AND found all the weird fairies in addition to the standard treasures. Time well spent. Wind Waker was short of a "real" dungeon, replaced by some half-assed "escape quest" in the beginning. TP has that insanely linearly Twilight Dungeon (I got to the goddamn "Twilight Realm," and this is the best they could come up with? Miyamoto must've laid the smack down to get this game done in time for Casual Gamers), an escape quest, and a handful of silly cinematic action sequences to make Link's adventure seem "cool" (and you knew what these were: an attempt to mask the empty space that makes up Hyrule Field, that you would suffer going back and forth through anyway later) and make up for the fact that Link can't inject much excitement into the MATURE ZELDA becuz he has no voice acting. /voice acting troll
Oh, and the completion time. Both WW and TP dungeons typically took me TWO HOURS TO CLEAR AND FIND EVERYTHING. LAME LAME LAME LAME LAME LAME LAME LAME rush job. You can tell Nintendo was running out of dungeon design ideas in WW and TP because every other dungeon was another fucking CYLINDRICAL TOWER. "Oh, where have I seen this before?" "Oh, I guess they want me to do this and this, and that will show me the boss door for sure" At least MM provided 2 of those (before making them stale on Nintendo GAMECUBE) and made you travel in ways you didn't anticipate; c'mon, we don't need another DEKU TREE.
MM was also the last 3D console Zelda to have LEGITIMATE SECRETS. None of the "map that leads to a chest that leads to ANOTHER MAP" bullshit found in Wind Waker. And none of the "Fifth Heart Piece" bullshit and "Heart Piece in an ARBITRARY DUNGEON TREASURE CHEST" bullshit found in TP. MM had only 4 primary bosses and thus 4 complete Hearts to give away -- meaning there were plenty of plain Heart-quarters to find, and they involved reasonable (to amazing) efforts to acquire.
Then ON TOP OF ALL THAT, we had the awesome sidequests people love talking about, bringing substance to the NPCs and their world that we haven't seen in Zelda since.
By the time I encountered the final boss, I had all the hearts, all the masks, all the ____ , the Oni Link Mask, and ~50 hours clocked in. That was time well spent. MM was a game for experienced Ocarina players.
and unlike TP's final boss, Majora wasn't a pushover. and unlike WW in general, MM's enemies and bosses did real damage.