Goku was injured and couldn't show his full SSJ power on Namek

[Piramid]

When Goku transformed into a Super Saiyan for the first time in front of Freeza, he was badly injured and could barely stand. Therefore, his true power as a Super Saiyan was much higher than what we saw on Namek.

A year later, when Freeza and his father arrived on Earth, Goku (in the original timeline) easily defeated them because he was actually much more powerful.

[Grimlock]

Yeah, but one correction:

• It's two years later. Freeza saga happens in AGE 762. Freeza goes to Earth in AGE 764.

[theherodjl]

I'd like to add that Freeza was also injured and had depleted a little bit of of his energy prior to jumping to his maximum state, so he wasn't quite at his best either. The Daizenshuu's numbers list their full power at 150,000,000 & 120,000,000 for SSJ Goku & Freeza respectively, but neither were likely at those figures for long, if they were at all due to injury & fatigue.
I imagine that Freeza didn't actually know what Goku's true power was as a result of him not being able to sense Ki and the both of them not being at their best, and so Freeza probably just settled on receiving a minor percentage greater than his maximum power for his mecha form. I put Mecha Freeza anywhere between 120,000,000 to 150,000,000, with SSJ Goku being around or somewhat above 180,000,000 after coming back from Yardrat.

[nickzambuto]

Freeza was equally injured from the spirit bomb. His tail was gone. Imagine getting squished under a tremendous weight so much that one of your limbs is lost. Like his tail must have been crushed or ripped off, it wasn't a clean slice. Surviving a spirit bomb is gruesome.

I also think the Super Saiyan form had some revitalizing effects to it. He was exhausted before but immediately gained his stamina back after transforming. So his injuries were not healed, but he was definitely flooded with new energy, kind of like an adrenaline dump.

I think Goku was temporarily at full power but he would have faded quickly if the fight went longer than 5 minutes.

[Piramid]

In addition to all this, I have the theory that Mecha Frieza not only became more powerful, but with his new body, he could also manage his energy more efficiently and could maintain his 100% indefinitely without getting tired like he did before.

[Koitsukai]

150 vs 120 doesn't make sense if you have eyes and pay attention to the fight in either media. I assume those numbers are hypothetical, if they were to be at FP, but in practice they probably were both around the 100 mark. Freeza gets to Porunga before Goku, impossible to do that if he's that much weaker.

[miguelnuva1]

Yeah, but one correction:

• It's two years later. Freeza saga happens in AGE 762. Freeza goes to Earth in AGE 764.

A year and half technically.

Goku fights Frieza Dec 24th 762.

Trunks fights Frieza Aug 764.

This was the biggest part of finding out Gohan should be closer to 11 vs Cell and not the 9 people pretend he is.

[Kaboom]

If all relevant official info tells us that Goku and Freeza were fighting at their respective full powers of 150 and 120 million, then I see no good reason to argue against it. It's not particularly far-fetched either — Freeza was still holding back most of his power all along, and Goku unlocking Super Saiyan would bring with it a brand new reserve of power that he didn't have before.

On the flipside though, it's still easy to assume that their injuries were physically holding them back somewhat, and effecting their actual performance in the fight. In the sense that they were both at full power but couldn't use that power as effectively as they could if they were uninjured.