The large icons to which your refer are not App icons - but are Widgets.
iPadOS15 has introduced the ability to add Widgets to any of your Home Screens - some being added to your first Home Screen, by default, during the initial update to iPadOS 15.x.
Widgets placed on a Home Screen now individually occupy one or more elements of a 4x6 icon grid. Home Screens without Widgets retain the familiar 5x6 icon grid. With the grid accommodating fewer icons when Widgets are present, some App and Folder icons may be displaced to the next Home Screen - and this may have a ripple-effect where Home Screens icon grids are fully populated.
More information about the new Widgets can be found here:
Use widgets on your iPad - Apple Support
The iPad User Guide is also a rich source of information - and is especially useful in becoming familiar with the many changes and new features introduced in iPadOS15. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892