Parker proposed the following evolutionary pathway to explain anisogamy. Assume isogamy, but some heritable variation in size.
Larger zygotes have greater chance of surviving because they have more stored food reserves.
Large gametes produce large zygotes. This produces immediate selection for small gametes to find and fuse with large gametes. Otherwise, they have no chance. Larger gametes should try to avoid fuzing with small ones, but the penalty is not as great. Intermediates lose out.