QtCreator displays structure sizes in tooltips when you hover mouse over the structure name in the structure definition. Those sizes are incorrect in a few contexts.
For example, in these definitions:
hovering mouse over A shows its size is 12 bytes, and B is 16 bytes. gcc shows 16 and 24, respectively (https://gcc.godbolt.org/z/YzGf5EeGn).
Here QtCreator shows 8 bytes for each struct while gcc shows 16 (https://gcc.godbolt.org/z/Wvnz6xd6f).
It looks like the struct sizes are calculated for 32-bit x86 target, even though the actual build target is x86-64.