When a Pages document includes images or video, the File->Reduce File Size menu at the top of Pages can sometimes help reduce the document’s size. See the “Reduce a document’s file size in Pages on Mac” Apple Support webpage for more details.
In general, however, a Pages document tends to be significantly larger than a Word file with similar content. The reason is that a Pages document is actually a folder with multiple files in it with metadata, a number of preview images and more. There is no way to permanently change that unless you save the Pages document in another format such as PDF, Word, Plain Text, etc.
To see that yourself, make a copy of a Pages file with a name different than the original (need not be a complex or large file at all). Then, rename the copy’s extension as zip and double-click on that zip file. This will create a new folder with the same name as the copy. Double-click on that folder and you will see a number of folders and files in it similar to that in the screenshot below: