One of the assumptions of a model that explains the evolution of anisogamy is shown in the figure ...

One of the assumptions of a model that explains the evolution of anisogamy is shown in the figure below. Why does this function not result in the evolution of isogamy with larger gametes?
 
  a. The number of large proto-eggs that can be produced is limited.
  b. Small proto-eggs are more likely to fuse with large proto-eggs than large proto-eggs, even if both occur at the same proportion and move at the same speed.
  c. Large proto-eggs are rare; so the chance of fertilization with common sperm-like proto-eggs is higher than fertilization with another large egg.
  d. Zygotes that form from the fusion of two small proto-eggs are common but short lived.
  e. A combination of A, C, and D.