Why is lead oxide a stronger oxidising agent than tin oxide?

There are two lead oxides and two tin oxides:

PbO2, PbO, SnO2 and SnO.

Lead(IV)oxide, (PbO2) is a stronger oxidizing agent than either tin(II)oxide or tin(IV) oxide because it has a higher tendency to be reduced, which means to gain electrons. A substance is reduced when it oxidizes or takes electrons from another substance. This higher oxidizing ability of lead(IV) ion is due to a reversal in a periodic trend.

The elements in Group 4A tend to lose 4 electrons, forming 4+ ions. The total ionization energy decreases going down the group until you get to lead. The total ionization energy for Pb4+ is slightly higher than for Sn4+, making Pb less likely to lose 4 electrons.

The Pb(4+) ion is unstable and is easily oxidized to the Pb(2+) ion. There's a phenomenon shown by the heaviest elements in which the electrons are pulled in closer to the nucleus and the two s electrons aren't easily removed. This makes the 2+ ion of lead (in which only the two 5p electrons are given up) more stable than the 4+ ion.