Leah's pretty intimidating: she had eight children at a time when having one child could be deadly. She's an octomom, folks. She also has eyes that can pierce into anyone's soul, she's physically strong, and she's the voice of reason in Jacob's family. Without Leah, Jacob wouldn't be the successful man he is for a large part of the book. She's the rock of the family—loyal, hardworking, and powerful.
By the way, Leah is also physically bigger than many of the other characters:
She was not only tall but shapely and strong. She was blessed with full, high breasts and muscular calves that showed to good advantage in robes that somehow never stayed close at the hem. She had forearms like a young man's, but her walk was that of a woman with promising hips. (1.1.30)
This lady's like a decathlete: strong in every aspect. And we're not just talking about her height or her muscles: even her "feminine" qualities stand out, as she has "promising hips" that would immediately attract men. There's really nothing more to say about Leah's physicality—she's simply a force to be reckoned with.
But Leah's not just physically strong. She's also the daughter of Laban, and she's the one who provides reason and intellect when it comes to decision-making. On top of that, she's totally in charge of the domestic work. If someone like Zilpah were in charge, the family would probably just end up praying for food and livestock until they all starved to death. Leah's the one who keeps things functioning.
When Jacob announces that the family is going to relocate, the daughters all have mixed opinions. But Leah's answer is no surprise: "'The Mountains will protect us against winds," said Leah, with reason" (2.6.11). If Leah were a student at a university, she'd probably be a business analytics major: cause and effect, calculations, supply and demand. She'd be great at making Power Points.
Our parents always tell us that when we shake someone's hand, we should look that person in the eyes. But apparently that never resonated with most characters in The Red Tent, since what most of them do is cringe when they catch a glimpse of Leah's stare.
Going along with her intimidating presence, Leah has eyes that can send a shiver down anyone's spine:
But my mother's eyes were not weak, or sick, or rheumy, The truth is, her eyes made others weak and most people looked away rather than face them—one blue as lapis, the other green as Egyptian grass. (1.1.24)
Dinah says that Leah's eyes could make "others weak and most people looked away rather than face them." Now, when Leah first meets Jacob, he stares directly into her eyes without looking away. For Leah, that's a huge plus. In her mind, this means that Jacob isn't like most men.
In general, Leah's dominant eyes just highlight her personality as a commanding female character. So it's fitting that it's Leah who ends up with Jacob first (and foremost)—as we can tell from their eye contact, these two seem evenly matched.