Totally understand them not being able to carry multiple people straight away. That can totally be the 15th level feature.
But at 7th level, it’s medium sized. Which, granted, is a wide spectrum. But surely it wouldn’t be too overpowered to allow the ranger conditonally permanent flight at that level, would it?
I’m currently playing a Drakewarden and they feel a little lacking in a few places. I don’t love that the dragon doesn’t get any stat bumps as it grows, like say getting stronger would be cool. They also don’t get any skill proficiencies which is baffling given the Tasha’s new Beast Master Stat block which seems way way better. a lack of flight until later levels feels pretty bad, and they do not get any kind of breath weapon until 11. You are a dragon rider and your dragon doesn’t have a breath weapon for most of the campaign, if you even get to 11 and lord knows how few people will ever see level 15 for flight.
Your first part is what hurts me the most about both companions, tbh. The lack of scaling attributes just feels off to me. I get why they don’t, but doesn’t mean I have to like it. I’d love to see something like:
“At X level, your companion has been steadily growing in power alongside you. When you gain this level or whenever you summon a new companion, that companion gains a +2 to any ability score and a +1 to a different ability score.”