I think the Windows 10 installer size will still be under 4GB of size in the near future, so it can fit on 4GB USB flash drive, and DVD-R single layer disc which only have 4.4GB of storage space.
A lot of people still use dial-up and other slower internet connection, so downloading a large Windows 10 installer file can be very slow on a slower internet connection.
Well if Microsoft changes the way they package their installer (they have for a good while now) the installer can still be smaller. Bear in mind though, the installer that has both 32 and x64 editions of Windows 10 is about 6.42 GB so it will fit on an 8GB USB and a dual-layer DVD.
There is also a possibility of Windows making future versions of Windows 10 installer smaller in size by removing bundled programs like Windows Defender, WordPad, MS Paint, Bing, etc because more users rather use 3rd-party software like Avast Antivirus, LibreOffice, InkScape, and Gimp, and make it possible to use a 4GB or smaller size USB flash drive to install Windows 10 on computers with 32GB or less storage space.
I feel someday Windows could become an online operating system like Chrome OS where most programs are stored on a Google's web server or are websites like Gmail, Google Docs, Google Drive, etc.
But, I feel users would get mad if Windows and bundled Windows programs like Notepad, MS Paint, Games, etc won't work if there is an internet outage or slow internet problems.
There is a possibility that users have to pay to download optional Windows software like Windows Media Player and Windows Defender, so Microsoft earns more money, and the download size is smaller in size sort of like free antivirus software where users need to pay money for extra features like a firewall, identity protection, and online file storage.
