If you define any market narrowly enough, you get a "monopoly". Walmart is a larger retailer than Amazon. You have to add "online" for Amazon to win. Is that valid, when most of Amazon's products are available at local retailers?
Also, the original landmark antitrust cases were against true monopolies, like Standard Oil with 90% of the world oil market, Alcoa with 80% of aluminum, or GM with (then) 60% of autos. Amazon has less than 40% of the US market, and a negligible percentage worldwide.
More importantly, Amazon has literally tens of thousands of competitors, ranging in size from thousands of employees down to mom-and-pop websites. A basic axiom of antitrust law is that it doesn't exist to protect competitors, but competition in general. The online market is about as competitive as it gets ... and as large as Amazon is, firms like Alibaba and Temu could potentially topple it literally overnight.
Then point out that "abuse" that's harming consumers. Amazon is large because it literally single-handedly reinvited online sales to remove consumer's primary problems with it. Before Amazon, an online sale meant waiting two to six weeks for your goods, sometimes longer -- and for smaller products, the dreaded "shipping and handling" fees could double the price. But with Amazon, I often receive orders within two hours of placing the order. Two hours! Shipping is free ... and they have the best return policy in the business.
Amazon is large because they provide a better product selection at better prices, with superior shipping and return polices. That's good for consumers. Not bad.