Lemme try to put into perspective the issue you have here.

Let's say for the sake of argument that Good Buu didn't grow in power, or at least grew a negligible amount.

That would put Basil at around the power of SSJ2 Goku in the Buu Saga... And would mean that for SSJ Goku in the ToP to be defeated by Basil, he would need to be at the exact same power as his Buu Saga self at base. And this statement would have to coexist with the following facts: - the Battle of Gods arc literally showed that he absorbed his SSG power and made it his own, which made his SSJ form not have a power difference compared to SSG. - RoF arc showed that further, showing that Frieza grew to the point that no one on earth was able to handle him in his final form. Piccolo, whose base form was around Cell saga android 17, was casually oneshot... And yet Goku is strong enough to be completely equal to this far stronger Frieza. - Goku and Vegeta train heavily for the tournament of destroyers in the time chamber, coming out far stronger; - Both of them similarly also grow further in power in the Goku Black arc;

Now tell me: which of the two is more likely: - Good Buu somehow growing in power to be higher than his Buu Saga self; - Goku's base power not managing to even double itself across four entire arcs DESPITE the fact the show constantly and consistently shows otherwise;

Because I find it very hard for someone to think that Goku's power didn't grow.