yes, you can adjust that. There are two options. To display the table toolbar, edit your page, then place the cursor somewhere inside the table. You will see on the left "Responsive" and "Fixed width".
When the table is "Responsive," you can't drag and resize the columns. Confluence automatically adjusts the column widths according the cells' content. "Fixed width" lets you set the width of each column by dragging it, and it will remain that width, independent of its content.
You can see how this works in Video #11 of our Video Tutorial series "Learn Atlassian Confluence".
In Atlassian's documentation you will find more info about tables on this page.