"There isn't enough space" is a good generic excuse when you want to weasel your way out of anything.

"Carry this load of rations." "There isn't enough space in my backpack."

"Scout the orc camp and tell us what you see." "There isn't enough space to squeeze between the trees."

"Open this treasure chest." "There isn't enough space for all of you to run if it's trapped with a fireball."

Transcript

Luke: Right. I've got a lot to think about. But R2 and I have to get back to the Rebellion.
Obi-Wan: About that: You'll have company. I hope that's okay.
Luke: You're coming with us this time, then?
Obi-Wan: We all are.
Luke: Not Yoda too? I don't want a crazy dead Jedi voice haunting me.
Obi-Wan: Not just Yoda. All of us transcended Jedi.
Luke: Um. There isn't enough space on my X-wing.
Obi-Wan: Sure there is. They all just got on.
Luke: <sigh> Why do they want to come anyway?
Obi-Wan: The Tree remains connected to Vader. Dagobah's days are numbered.
Luke: Great. A shipload of dozens of Yodas.
Obi-Wan: Oh no, not at all. There are thousands of us.