Chemistry: Acid Strength

Hey guys, I had a general question regarding acid strength. When comparing two acids to find out which is stronger, two factors are involved: bond strength and polarity. The weaker the bond strength the easier it is for the acid to lose its hydrogen atom and the stronger the acid is. The more polar the molecule, the more positively charged the hydrogen is, and thus the stronger the acid.

Generally when compare two acids with elements going down a group of the periodic table, bond strength is the determining factor. When going across a period, electronegativity (the factor that determines polarity) is the determining factor.

The question I had was determining which of the two acids : $H_2SO_3$ or $H_2SeO_3$ was stronger.

Since we are comparing acids whose main elements are in a group together, I thought that bond strength would be the determining factor, but apparantly the determining factor is electronegativity.

Could anyone explain why this is so?

P.S. Bond strength depends on the size of the central atom. The larger the atom, the weaker the bond, the stronger the acid.