TT #688 · Parashas Ki Seitzei · Seventh Aliyah · Passuk 25:06
And it shall be that the firstborn that she bears shall succeed to the name of his dead brother, so that his name shall not be blotted out from Israel.
Torah Temimah points to Bekhorot 52a.
TT lemma: וְהָיָה הַבְּכוֹר.
כַּבְּכוֹר – מַה בְּכוֹר אֵין לוֹ בְּשֶׁבַח שֶׁשִּׁבְּחוּ נְכָסִים קֹדֶם חֲלוּקָה, אַף יָבָם אֵין לוֹ בְּשֶׁבַח שֶׁשִּׁבְּחוּ נְכָסִים קֹדֶם חֲלוּקָה.
English: Like a firstborn — just as a firstborn has no share in the appreciation of the estate that occurred before division, so the yavam has no share in the appreciation that occurred before division.
On the daf / in context: The comparison of the yavam’s portion to the double portion of a firstborn limits his rights in certain increases of the estate.
Analogy to the firstborn’s inheritance rights.
The yavam’s inherited share does not include appreciation of the estate that accrued before the division of the property.