Why would a group of small cells an organism maintain homeostasis better than one large cell

Because the group of small cells would have a greater surface area-to-volume ratio, so they are able to exchange materials with their surroundings more efficiently.  In shorter terms, they have a greater surface to volume ratio. 
A critical part of homeostasis is the tranfer and intake of molecules (whether necessary or not). It is important for the cell to be able to bring necessary nutrients immedriately where it needs to and to be able to dispose of harmful byproducts of the cell process.
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Fossil found in strata reveal that unused body part decrease in size