In the Mahayana in general, the bodhisattvas are viewed as higher than a buddha because they put off full enlightenment and parinirvana unless/until all sentient beings can be saved. You have to suffer for the sake of the world in order to be a perfect being.

In the Lotus Sutra, it goes so far as to argue that the Buddha didn't actually attain parinirvana but instead is himself a bodhisattva. Otherwise, it would imply that the Buddha went down an inferior path (that of the sravakas or pratyekabuddhas) rather than the Mahayana/bodhisattva path.

The Lotus Sutra teaches that there is really only "one path" (the Mahayana/bodhisattva path) but that the masters have taught three paths (sravaka, pratyekabuddha, and bodhisattva) as a "skillful means" to draw people in and push them in the right direction. You can start with self-interested reasons but eventually turn into a bodhisattva. The idea of being able to attain parinirvana through other paths (to become enlightened without embracing suffering and staying in the world for the sake of others) is sort of a white lie. The Lotus Sutra argues that the lie is justified by giving a parable of a father whose house catches fire. His young kids are playing inside and they don't understand what a fire is. There's too many of them to drag out of the building by force. So he ends up telling a white lie, as a "skillful means," in order to get them to leave the house and go to safety of their own volition. He tells them that he has gotten them a bunch of toys, and horse-drawn chariots, and all sorts of amazing things, so they need to go outside the house and outside of the town's gates to see all the cool things. So they rush out of the building and to safety on the false hope generated by a false promise—and this false promise is likened to the idea that you can attain enlightenment in any other way than through becoming a bodhisattva. But then it turns out that the white lie ends up not being a lie at all because the father later goes and buys the kids all the things he promised them. In the same way, the lie told by the Buddha is a little white lie that serves as a temporary "skillful means" that pushes hearers in the right direction, but ultimately every buddhist must go down the bodhisattva path because it's the only path (this is the Lotus Sutra's doctrine of the "one path"), but the lie ultimately isn't even a lie because full-enlightenment is at the end of the bodhisattva path (the Mahayana path).

The Lotus Sutra is the most influential Mahayana sutra and the Mahayana is the largest branch of Buddhism.