Startups generally offer more freedom and less micromanaging. This also means that you’ll necessarily receive less guidance, and will need to be able to figure stuff out, learn, and make progress under your own power.
In a big company, you’re likely to experience more structure, and be expected to follow very clearly defined pre-existing processes. Your job scope will likely be more focused than it would be at a startup. You’ll experience less freedom in general, but also more certainty in what’s expected of you.
In the end, especially at the beginning of your career, don’t put too much stock in choosing one or the other. If you like the company, big or small, give it a try. If you’re not happy there after a few months, then try another one. No career decision is ever permanent.