Well yes, we had low resolution images, no consideration for different viewports, no non-default fonts and little interactivity beyond links or other queries to the server.
Responsivity is a few hundred bytes of CSS. Modern formats for fonts (woff2) and images (webp) allow you to squeeze them in a couple hundred kilobytes, which still leaves plenty of room for scripts.
For e.g., if someone uses Google Analytics, that alone comes to 430kb (which most people do)