The 10MB file is just a stub installer. When you run it, it connects to a server to download the actual 4GB of files. That isn't "highly compressed"—it's just a redirect. Often, these servers are slow, insecure, or shut down after you download the stub.
Every day, thousands of users type this exact phrase into Google, Bing, or torrent sites, hoping to dodge a $150-$400 price tag and a 4GB download. But does this file truly exist? And if you find a 10MB installer claiming to be Microsoft Office 2021, what exactly are you downloading? microsoft office 2021 highly compressed 10mb