realloc will grow if possible, but will malloc and copy if it cannot be grown. Some allocators will round up to a larger size (like a page size), so allocations up to the page boundary will be free. Most modern allocators use small block optimizations and other techniques, the net effect of which is that realloc will usually require a malloc/copy.

You should treat realloc as a semantic hint to the allocator, but for performance analysis you should always assume that it will allocate/copy.