Why is Golden Frieza considered stronger than regular Frieza when his Golden Form did not significantly increase his power level?

Who said that?

This is Final Form Frieza a bit inferior to Base Goku.

This is Golden Frieza slightly superior to Super Saiyan Blue Goku.

Now as far as the Resurrection F movie is concerned, you are somewhat right in that Goku was fighting Final Form Frieza using the Saiyan beyond God form which is Super Saiyan God in Base and then transformed to Blue which is a Super Saiyan multiplier on top of God.

So Golden Frieza should be slightly superior to the Super Saiyan Form as a Multiplier which is still quite a difference like 50 times or more.

However, Dragon Ball Super didn’t include the Saiyan beyond God form but it was just Base Goku after the Beerus fight who does scale to the Super Saiyan God form but not because of any God Ki but with mortal Ki alone.

This means that transforming into Super Saiyan Blue is easily thousands of times multiplier since we have the 400 times to Super Saiyan 3 and then the humongous difference between Super Saiyan God and 3 which is tens to hundreds of times and then Super Saiyan Blue which is a Super Saiyan level multiplier over that.

Goku vs Jiren shows the difference between the forms quite accurately if you don’t want to go too deep into numbers.

Base Goku is so inferior he can’t even get near him.

Super Saiyan Goku can get near him but Jiren tanks every attack. Same with Super Saiyan 2 who only manages to move his head a few centimeters compared to Super Saiyan which did nothing at all.

Super Saiyan God forces Jiren to actually block but with just one finger. Hey that’s progression.

Super Saiyan Blue actually manages to make Jiren fight like he’s facing an actual opponent, Goku’s punch pushes Jiren back through the rocks even when he blocked with his arms and he started going on actual offensive so like huge power difference between God and Blue.

So if you’re thinking a Super Saiyan multiplier isn’t much, this scene beautifully displays the difference which clearly shows that even in Resurrection F film, the difference between Golden Form and Final Form is steep but in Super anime it is Ginormous that he goes from Base Goku to stronger than Blue.

It’s absolutely wild when you think about the difference. When he mastered his Golden Form in Hell, he became an Equal to Goku again even though the whole Goku Black/Zamasu arc happened. The Manga makes it look even greater because Perfected Super Saiyan Blue Goku was equal to Fused Zamasu like it’s that high of a multiplier enough to be a fusion level multiplier and this is Goku when fighting Golden Frieza.