Phase I and Phase II were built a different times and are administered by 2 different HOA sub-associations. The Master Association administers the resort as a whole.
The allocation of responsibility for expenses among the 3 associations is governed by the Declarations of the 3.
If a Phase III is ever built, there will likely be a 3rd sub-association.
I have not looked at the documents, but the Master Association would usually administer everything pertaining to the common property (property that can be used by everyone). That would be everything except the envelopes of land for the condo buildings and the buildings on that land. In other words, upkeep, staffing and replacement of the landscaping, the pool, the restaurant, the reception area, the roads, the work out room, the security at the main gate, the shuttle buses and the sales office that is probably rented from the Master Association by Starwood.
The sub-associations would handle the replacement of the condo buildings, the maid service for each and the furniture in the units (and maintenance) in their Phase.
In general, the Master Association expense is the allocated cost of running the Master Association for the benefit of the owners of both associations. That's why it is large.

... eom