I'm not sure this fits with the hint, but are you
I lie between a tennis ball and a soccer ball
Ceres, the dwarf planet the element is named after is between Mars and Jupiter, the size of Mars as compared to Jupiter is similar to a tennis ball and soccer ball.
I agree that I'm small.
An atom of Cerium is very small indeed.
I am at the bottom but at first,
Cerium is in the lanthanides, which are at the bottom of the periodic table, but in the first half of the first row. It was also the first lanthanide to be discovered.
Sometimes, I may even burst.
Cerium is pyrophoric, and used in a specific type of lighter.
My prefix rhymes with an app,
"Ceri-" sounds like Siri.
I'm in Earth, but not in its map!
Cerium is a rare earth element.
Find me from a cluster,
Can you, mister?
Not sure on this one, Cerium is named after the dwarf planet Ceres in the asteroid belt?