size vs capacity of a vector?

Size is not allowed to differ between multiple compilers. The size of a vector is the number of elements that it contains, which is directly controlled by how many elements you put into the vector.

Capacity is the amount of total space that the vector has. Under the hood, a vector just uses an array. The capacity of the vector is the size of that array. This is always equal to or larger than the size. The difference between them is the number of elements that you can add to the vector before the array under the hood needs to be reallocated.

You should almost never care about the capacity. It exists to let people with very specific performance and memory constraints do exactly what they want.

Size: the number of items currently in the vector

Capacity: how many items can be fit in the vector before it is "full". Once full, adding new items will result in a new, larger block of memory being allocated and the existing items being copied to it