There are two factors that affect what fraction of your budget is spent. First is the total inventory of available impressions, that is, how often consumers search for products related to the products you advertise.
Second is the competition for product placements on these pages. You cannot control total traffic on the website via your ads. But you can control how competitive your bids are. If you are willing to spend more to capture additional impressions and clicks from your competitors, you can increase your max bid.