Several points here:
First of all size / trend is the most common trap in the cases. Candidates often forget asking about the size / trend and get the wrong conclusion. Always ask for:
- Size of the market and its growth rate
- Size of the segments and their growth rates
- Shares of the competitors and their growth rate
- Revenue of the company and the growth rate
It's absolutely critical to discipline yourself to get the information about the size and the growth together!
Answering your specific question - both are important but growth may be more important in immature industries (e.g. the start of the telecom market when penetration was low, or ridesharing market when Uber just launched.) In other words, growth may be more important when you are creating the new market / submarket