C++ vector size to int conversion warning

I have just started learning c++, and I found an strange behavior while assigning vector size to int.

When I do like below, Then there is no warning

int removeElement(vector<int> &nums)
{
     const int len = nums.size();
}

But when I write code like this, then it gives me warning

 warning: narrowing conversion of '(& nums)->std::vector<int>::size()'
 from 'std::vector<int>::size_type' {aka 'long long unsigned int'} to
 'int' inside { } [-Wnarrowing]

The erroneous code:

int removeElement(vector<int> &nums)
{
     const int len { nums.size() };
}

I want to understand, why there is no warning in first case?