Directive @media SCSS and @extend which is correct?

Question

Sass

Good afternoon,
there is such scss

.titles{
@extend .size50;


 
@media (max-width: 992px){
@extend .size20 ;
}
}

respectively at the beginning

.size20{
font-size:20px;	
}
.size50{
font-size:50px;	
}

But still, at a resolution less than 992px, the font size is 50 px. The visibility area does not work along the way ... tell me how to correctly make a media query. Thanks