windows 10 update has a 86 Gb file to download

My 64-bit Windows 10 installation had recently updated itself to April 2018 update through Windows Update. I have recently started using Windows Update MiniTool.

That is showing that Windows 10 update has a 86 GB file to download

it is reading -
- Title: 2019-01 Cumulative Update for Windows 10 Version 1803
- KB Article: 4480966
- Size: 86.95 Gb
- Date: 1/8/2019

My Windows 10 partition is just 100 GB. Windows 10 is currently using approximately 20 GB. I also have a previous installation rollback which is consuming an additional 20 GB.

Windows Updates does not show the update file size.

How can a 96.95 GB file be required for a 20 GB installation? It just can't download and keep that file in my 100 Gb partition. Something is stuck. What am I missing?

UPDATE: 16 Jan 2019
issue 1.
Today I went to windows 10 again to download that update. I unhide that from wumt so that windows update can see that. windows update started downloading that and counter rushed to 25% (probably meaning that this much was already downloaded). Then after, I wend up to some 33%, and had consumed my 1.4 GB net quota for the day. Then net stopped so update stopped.

No other major net activity was done in this duration that would have consumed major net quota.

That means the update is surely not 800 MB or so as was suggested here.

if 1.4 GB net downloads 33-25= 8% file, it can be calculated what the size of file could be.

issue 2:
I gave run - winver, and that said "Version 1803 (OS Build 17134.112)"

meaning 1803 update is already installed, that it had downloaded in several days and my w10 look is also different from what it was earlier on pendrive install.

So, when 1803 is already installed, is this a revision of that update that it is downloading now, or is it stuck somewhere.

Any method to stop it from repeat download of 1803 if this download is not different.

issue 3. I read on net that 1809 update is also released way back in Oct 2018, why is windows update not showing and downloading that? Will that appear after 1803 is installed?

Thanks.

Answer

How can a 96.95 Gb file be required for a 20 GB installed os?

The update in question is not actually 86.95 GB. The actual size of KB4480966 is between 448.8 MB (x86) and 802.7 MB (x64).

it just can't download and keep that file in my 100 Gb partition.

You actually could, since the actual size of the file, is between 448.8 MB and 802.7 MB. There is no possible way, outside of the tool misbehaving, for the update to actually be 86.95 GB.

What am I missing?

You are using a third-party tool, which is no longer being developed by it's author, and which clearly has an undocumented bug.