The issue isn't what is the largest blood type, it is who can be a donor and who can be a receiver.
Setting aside positive and negative blood types .... O is the best "donor" type while AB is the best "receiver" type.
Someone with an O kidney can donate to O, A, B or AB
Someone with an A kidney can receive from an O or A kidney
Someone with a B kidney can receive from an O or B kidney
Someone with an AB kidney can receive from an O, A, B or AB kidney
But, someone with an O kidney can only receive from an O kidney.
So even though O+ is the largest blood type, he has a smaller pool of available donors to find an available kidney from.