So glad you posted about this as I've been having the same issue. DD was 1 ounce shy of 10 pounds at birth so a big girl and the infant insert always made the ergo feel funny on me. Not so great funny. Like it pushed it out too far which made my body kinda curve in and stick out in unnatural, uncomfortable ways. But I was worried about her legs so I kept using the insert until about 10 weeks though this limited my ergo usage to only when necessary, like long walks not just around the house. But then the ring sling was getting hard on my body and she was starting to reject it and I saw another young baby in his ergo with no insert and so felt I had permission to break the "law" of 3-4 months and go insert-less.
Long story short, DD's still standing or froggy-style in it and she just turned 3 months yesterday. Her legs sometimes splay that far when she has a cloth diaper on but she protests if I put them that way in ergo and I agree with her, seems just a little too far at this point. She's somewhere around 18 pounds and over 24 inches so not sure if height is the secret or just individual baby readiness? And I do worry about forcing her hip joints. The only problem I have with froggy or standing style (apart from feeling like I have to hold her in at some moments) is that her head will be really high up, like touching my face, if she's not in there right or it's not tight enough etc. Usually I can shimmy her around to make it work but I'm hoping that she'll move to splay before she gets much taller!