I'm sorry for the confusion. Some references (older ones) place more emphasis on atomic size, and therefore claim that perbromic acid is stronger than perchloric acid because of this.
However, more recent texts claim the other way around, that perchloric acid is stronger than perbromic acid, due to electronegativity.
It's tough to say, merely because our trends don't always work out in more complex cases such as these.
Older texts, especially those printed around 1950-1960, as well as recent texts that reference these older ones, claim HBrO4 as the stronger of the two. It turns out that around that time period, 50s and 60s, HBrO4 had not even been isolated yet. So the attempts at understanding the acidity of such a species relative to some other known species was quite theoretical.
So there's a conflict with resources, whomever you get the information from. And since current sources often cite these "archaic" references, there's bound to be confusion.