Fast, lightweight models suit clear, low-risk tasks: summarising a meeting, sorting emails, reformatting data, drafting first versions, or pulling fields out of documents. They respond quickly and usually cost less, which matters when you repeat a task hundreds of times.
Move to a larger model when the task needs multi-step reasoning, careful reading of long or messy documents, nuanced writing, or when a wrong answer is expensive, such as legal, financial or production code work. A simple test: run ten real examples of your task through both models and compare the results side by side. If the small one passes nine, use it and keep the bigger one for the exceptions. Model lineups change often, so check the vendor's current documentation.