Electronic file size is the measure of space an upload, attachment, or document takes up on a computer (not only driven by word or page count). File sizes can be measured in small kilobytes (KB), large megabytes (MB), and beyond. Just 1MB can hold 500 pages of text, so, usually, a large file size is caused by the way the document was saved or scanned. It is increasingly important in most transactions today to be familiar with file sizes as a courtesy to clients, colleagues, and others when you send documents, pictures, etc. to their inboxes. Everyone pays for data and storage, and items you send to others use up some of their bandwidth and allotment. Take notice and try to limit when a file you are sending is above 1MB as a general courtesy; most everyday text-heavy files can be much smaller than this without losing quality.
Some scanner machines are set by default to take very high resolution scans of your materials, producing very large file sizes no matter how few pages they contain. If you scanned your file and it is now several megabytes large, your scanner settings can be adjusted to be lower-resolution from now on, saving yourself and others storage. For documents you already scanned, you can use an online compressor like https://www.adobe.com/acrobat/online/compress-pdf.html to make them smaller now. And, for other types of files, there are many learning resources like this online where you can read about best practices for creating files of manageable sizes.