Different results in responsive design modus and when it is turned off

I feel silly asking this but anyway, writing media queries I see what the browser does by reducing the width of the page. But, when I use the dev tools and I click on the responsive icon (you know those two overlapping squares in the top right corner of the dev tools) to put the page in responsive design modus I get different results. Also, that responsive design modus is nice but it is not always 100% correct. So, if you don’t want to pay anything, what do I do. Which is more correct, using responsive design modus or just playing with the width of the page? I have placed a big title next to a figure and both need to grow smaller gracefully. When the font grows smaller I have to adjust top and left values and there seems to be no solution that looks nice in both responsive design modus and when that is turned off. That title is either too high or too low