For a variable to be growable (so that it can occupy different amounts of space in the memory at different times), it needs to be allocated on the heap, and not on the stack. The stack works faster, since on the loading of the program, it gets assigned to it. But the heap is slower, since for each allocation you need to perform a system call to the kernel, which means you will need a context switch (to kernel mode) and back (to user mode). This makes things too slow.
Vectors (and other standard library structures) have an interesting way of allocating that memory so that they perform as efficiently as possible. Let's check the algorithm it uses to allocate new memory with this code:
let mut my_vector = vec![73, 55]; println!( ...
