"Market share" means the percentage that a company or product sells compared to all of its competitors. For example, if an athletic shoe company has 30% market share, it means that 30% of the money spent on athletic shoes goes to that company.
When people describe "market share", they use size words like "large", "small", "bigger"etc.
They have a larger market share than all of their competitors combined.
When you want to describe how a company's market share is changing, use "grow", "shrink", "increase", or "decrease":
Our market share has been shrinking for the last few years.