This phrase means that the objects being referred to are so large and heavy that it is not possible for them to be able to climb trees.
When someone says "they are too big and heavy to climb trees," they are suggesting that the individuals or objects being referred to possess certain characteristics that make it impractical or impossible for them to maneuver and ascend trees. This could apply to animals, people, or even objects that are physically large and heavy. The statement implies that the size and weight of these entities make climbing trees challenging or unfeasible.
