Ugly font when compiled application using Qt 6 in Linux

Question

I compile a simple QT GUI application with Qt 5.15.2 and Qt 6.2.4 in Linux Ubuntu 20.04.4. As you see in the below figure, the font of the application compiled with Qt 6.2.4 (Right Fig) is bigger and uglier than the program compiled with Qt 5.15.2 (Left Fig).

Point: I do not use the following code:

QApplication::setAttribute(Qt::AA_DisableHighDpiScaling);

or

QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

Point: Compile sample:

g++ -c -pipe -Wno-unused-function -O2 -std=gnu++1z -Wall -Wextra -D_REENTRANT -fPIC -DAPP_TARGET