The way in which it is split should be set out in your lease. it may be fixed share a 1/3 or 66% or a fair proportion.
As the freeholder you most usually have to insure the building and recover the cost under the lease. In that case the insurer should not be refunding any money, they have been paid their premium, how you and your leaseholders divvy it up are up to you.
What does your lease say? That's the starting point.
If it is not fixed then the usual way is on floor area of the units with a slight weighting to the renewal costs of fixtures and fittings in the smaller flat, and any other circumstances.
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers.