1)
Why does it say "v'Yadata Ki Rav Zar'echa"?
1.
Devar Eliyahu: This is the opposite of a Rasha "Yichbedu Vanav v'Lo Yeda." Every added dearness of a Tzadik is known.
2)
What is the difference between Zar'echa and Tze'etza'echa?
1.
Devar Eliyahu: Zar'echa are sons; Tze'etza'echa are grandsons. The latter are compared to Esev ha'Aretz, which grows by itself without seeding. Zar'echa (sons) are compared to Zera - the father needed to seed it. Zar'echa is singular, without a Yud, but Tze'etza'echa is plural, for many children could be born at once.
2.
Malbim: Tze'etza'echa are Ketanim - they multiply like grass
