C++ :: How To Concatenate Two Vector Char

This seamed as a simple thing but i am getting something i did not expect: Example:

Code:
vector<char> StrJoin(SubjSeq.size()+ QuerySeq.size());
cout << StrJoin.size()<<"
"; // size x

StrJoin.insert( StrJoin.begin(), QuerySeq.begin(), QuerySeq.end() );
StrJoin.insert( StrJoin.begin(), SubjSeq.begin(), SubjSeq.end() );

cout << StrJoin.size()<<"
"; // x*2

All structures are vector<char>. when i do the above my characters form Query and Subject are copied in my new vector called StrJoin but the size of that vector is twice the size then it should be.