Mobile-First Solution using css flexbox

I had serious issues at the very very last few minutes adjusting to get the font sizes and different divs looking right. the transition from mobile to desktop made all sizes completely wrong. is it normal that your media query has a ton of small details to make things look nice, or should I not worry about that version looking just like the picture after the transition as long as it transitions smoothly? or is there something I am missing completely that could make the sizes look good on both sides?