Exhibiting parsimony; sparing in expenditure of money; frugal to excess; penurious; niggardly; stingy.

having extreme frugality. In regard to phylogenetic trees, having the fewest assumptions or branches.

Stingy. Although in normal language, this has a negative connotation, when applied to a model or an explanation in economics it tends to be positive, meaning that it relies on as simple a structure as possible.