Yes, big companies can copy your product. But at what cost, especially when including opportunity cost and the levels of bureaucracy in larger companies? I have heard your question many times during the last 20 years, and still there are more software startups.

In many cases it would cost them less to buy your company than to create it from scratch. Which is why so many small companies are acquired instead of copied. The question should be more what do you want to do and are you able to follow up on creating a startup? Writing the code is a small part of creating a successful startup