Nitrogen inversion barriers of trimethylamine and N,N-diisopropylmethylamine Which would have a smaller nitrogen inversion barrier: $\ce{NMe3}$ or $\ce{N(i-Pr)2Me}?$


I think that $\ce{NMe3}$ should have a smaller inversion barrier as it is less bulky, but data shows otherwise. Are there some other effects that contribute to the inversion barrier that I might not know of?
So I thought about this for a bit and I think I have an answer. Be warned, I might be wrong.

The inversion process involves a conversion from $\mathrm{sp^2}$ to $\mathrm{sp^3}$. So $\mathrm{sp^2}$ hybridization increases angle between these bulky groups, which would be in fact favorable. Hence the low activation energy.