It is still a child. I don’t think it translates
1:1 with human aging, but this species has a particularly long infancy-early childhood stage.There might be something unique to the Yoda species regarding Force sensitivity, but it’s already been established in canon that Force-sensitive babies in general have a predisposition to using the Force, regardless of species. The idea being, I’m assuming, that babies and young children lack some of the qualities that adults have that can hold back their ability to use the Force (like doubt or disbelief). And it is possible that the time he spends in this young childhood stage plays a part in why he can lift a mudhorn vs a toy ball.
You say that Luke could not stop and levitate the Rancor because he did not believe he can do it? Despite that Yoda taught him the opposite?
And what about Anakin and Obi-wan? They could not stop the charging monsters either.
And if this babyYoda is so much stronger, then how come that no Jedi ever sensed it’s presence in the Force, nor did Palpatine who sensed a seemingly much weaker Luke.I mean, Yoda tells Luke that “size matters not”, and the difference between lifting a rock and a ship was “only different in your mind”. So what is the difference between a ship and a rancor?
Yoda and Obi-wan are lying liars who lie. As the Mandalorians say “It is the way.”
And if you watch carefully you can see that size matters. Weight matters.
Yoda needs to concentrate hard to lift that X-wing. Do you think that it would take the same amount of effort to levitate a rock? Luke certainly does not, as he can levitate rocks and Artoo, but can barely raise the X-wing a bit.
And in another scene Yoda really struggles with that huge metal thing that Dooku throws at Obi-wan and Anakin.
So if it would be true that “size matters not”, these things would not happen.
The explanation here is that Yoda merely tries to motivate Luke, so he can believe in himself, which is very important for being able to do very difficult things.
Good coaches do this all the time.
To answer your question, the difference between a ship and a rancor in this regard is nothing. Luke can’t do neither.
The problem is that babyYoda can, which makes it one of the strongest Force user in the galaxy. Only one of the strongest, because I don’t know if Yoda or Palpi could stop and levitate a charging monster. Vader/Anakin could not.
And as I said, knowing the circumstances this is impossible.