Short answer. The share of the heir who drops out. In a reciprocal substitution among heirs instituted in unequal shares, the substitute acquires the share of the heir who dies, renounces, or is incapacitated, unless the will clearly shows you intended otherwise. Where there are several substitutes, they take it in the same proportion they held in the institution.
What the law says
If heirs instituted in unequal shares should be reciprocally substituted, the substitute shall acquire the share of the heir who dies, renounces, or is incapacitated, unless it clearly appears that the intention of the testator was otherwise.
Civil Code, Article 861 — Reciprocal Substitution. Read the full provision →
The vacant share passes as it was
Article 861 answers the puzzle that arises when heirs given different shares are made each other's substitutes: if heirs instituted in unequal shares should be reciprocally substituted, the substitute shall acquire the share of the heir who dies, renounces, or is incapacitated, unless it clearly appears that the intention of the testator was otherwise. The rule is that the substitute takes the share that fell vacant, exactly as it stood in the institution, not a share equal to his own and not some averaged figure. If the heir who drops out held a larger portion, the substitute receives that larger portion on top of what he already had.
A worked illustration
Suppose you institute one heir to two-thirds and another to one-third, and name them as each other's substitutes. If the heir given one-third falls out, the other acquires that one-third in substitution, on top of his own two-thirds. If instead the two-thirds heir falls out, the one-third heir acquires the two-thirds. The substitute inherits the departing heir's actual share, so the unequal proportions you set are respected rather than flattened. The size of what passes depends on which heir drops away, not on the substitute's original portion.
When more than one substitutes
The article adds a rule for the case where a vacated share is picked up by several substitutes at once: they share it in the same proportion they held between themselves in the original institution. So the relative shares the testator originally chose carry through into the substitution rather than being reset to equality. This keeps the whole distribution consistent with the balance the will struck at the outset, extending the testator's chosen proportions to the redistribution that a substitution triggers.
Your contrary intention controls
All of this yields to a clearly expressed different intention. If your will shows you wanted the substitute to take something other than the departing heir's exact share, that governs, but the intention has to appear clearly, not be merely arguable. So if you mean the substitutes to divide a vacated share equally regardless of their original proportions, or to take some other measure, say so in plain terms. Otherwise the default applies: the substitute steps into the precise share the fallen heir would have taken.