Usage of adages like "Bigger is always better" in this way may be considered a faulty generalization, and more specifically a "thought-terminating cliche":
a commonly used phrase, sometimes passing as folk wisdom, used to quell cognitive dissonance, conceal lack of thought-entertainment, move on to other topics etc. but in any case, end the debate with a cliché—not a point
"List of fallacies". Wikipedia.
A second issue that you point to is not a fallacy, but a concept known as "diminishing return":
Also called law of diminishing returns. Economics. the fact, often stated as a law or principle, that when any factor of production, as labor, is increased while other factors, as capital and land, are held constant in amount, the output per unit of the variable factor will eventually diminish.
any rate of profit, production, benefits, etc., that beyond a certain point fails to increase proportionately with added investment, effort, or skill.
Definition 2, is more ambiguous and allows for broader application, to describe when the increase of some factor does not yield in a significant increase in the overall value (i.e. Wearing shoes on hot coal stops your feet from getting burnt. Person A has more pairs of shoes than Person B, so Person A's feet will be less burnt than Person B.). In Philosophy this idea has several applications; e.g. to contend with the concepts of maximization of Utilitarianism, or John Rawls's discussion of distribution related to Social Justice.