Nintendo games are as big as ever. It's just the hardware userbase that's limiting them. If the NX does better, the games will do better.

If you look at how many of the WiiU owners that gets theese games.. Its a pretty easy qustion.. sell more hardware.. :)

Stop being so cheap with them.

I think they are still big, just not as relevant in kids and some peoples everyday life like they used to be. Mario Kart 8 has had really good sales despite its userbase, Same with super mario maker. The love for Mario games definetly still exist.  Legend Of Zelda i think is becoming bigger than ever. We get constantly new Zelda games and things like that, with spin-off titles, remakes and so on and so forth. I think if the NX has a big userbase we will maybe see Zelda titles getting big sales. Also Zelda remakes/remasters on the 3DS sell really good.

What i want Nintendo to do though is make Metroid more relevant than ever before, and not the way like Federation Force, (it still might be a good game, but not what will attract people to the metroid franchise). Nintendo really needs to maybe remake or just make a brand new full blown awsome Metroid game, with different exotic planets, haunting atmossphere and the feel and mood like the Metroid Prime trilogy blended in with Super Metroid

^Boom. 

All Nintendo franchises on the Wii had at least one game that sold in the top 5 of their franchises.

They need a bigger userbase. Games like Xenoblade X aren't that profitable when your userbase doesn't reach the 15m. And of course, Nintendo knows how to make big money with small games (Pokemon games, Mario, Splatoon...).

I think it all comes down to the kind of audience they target. Nintendo as a company have stayed focused either on the traditional younger gamers that like Mario or more niche, hardcore experiences like Xenoblade style RPGs, maybe some horror games here and there.
TBH Mario Kart still moves big numbers, though Nintendo could do with making new IPs that target the western 3rd party audience, since that's where most gamers are.

It's the lack of games that could interest 3rd party gamers, exclusive to a Nintendo platform that really prevents gamers from buying into a Nintendo system now.

Stuff like The Witcher, Gears, Uncharted, God of War. More shooters, genres that are a bit more grounded in reality, instead of fantasy would help to move platforms and that would have a knock on effect in how many people buy other Nintendo games.
Maybe Mario and co would sell even better if Nintendo had more new IPs that bring the western 3rd party gamers to their system, because those people would already own the system and may be interested to try something different.

It might help to make a new Metroid/F-Zero/Punch-Out!!/Kid Icarus/Warioware/etc. more than once every 5 to 10 years, if that. Otherwise, people get the impression Mario and Zelda is all they have.

Um...they are quite big. Albeit the game industry has shifted. But still, Nintendo games aren't small by any means. Especially if we're talking handheld sales. But to answer your question, to make them as big as they used to be it needs a better console install base. Which requires a console powerful enough to run 3rd party games with ease.

Nintendo hardware is limited by the lack of general appeal of their games.  if nintendo doesn't improve their games to appeal to a wide range of gamers the NX won't do any better than wiiU did.

software sells hardware.