Why is “but koalas are lighter” a subordinate clause in this sentence?

This is exactly how the sentence is presented:

If you ask me, pandas are far cuter than koalas but koalas are lighter.

So I was required to split this into its clauses, and categorise them.

“If you ask me” = Subordinating Clause (the comma is the sub. conjunction.)

“pandas are far cuter than koalas” – Main Clause

“But koalas a lighter” – I said that this was a Main clause as it is connected to a main clause by the conjunction “but”, which I thought would be a coordinating conjunction, hence suggesting that that clause was a main one.

However, the textbook says that that is actually a subordinate clause.

Why is this the case?