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It doesn't matter if the via size is bigger than the distance between pads, or the ball, if in fact the via does not need to be there. All that does matter is whether the via is bigger than the ball sitting on top of it, and enough so to touch the next ball over.

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Note that for mass production, you can't put a 'standard' via directly under a BGA/WCSP ball, as the via will 'suck away' the BGA ball via capillary action. You'd need to ask the board house to plug the via, which likely brings it into the 'expensive' range of things again.

OK, reading more carefully, I see that the actual minimum via diameter on that board process is too big. It is sort of understandable if they need room for the interior plating. That is very tiny!

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