Saving a Small jpeg file for web use - but it's over 6Mb!!!??

Hello

I'm coming on here as depite several hours on the phone to the horrendous adobe support we are still no further forward and this is driving me mad!

So, I would describe myself as a fairly advanced user of Photoshop. I know my way around. However since the recent Mac OS change to Monterry and Adobe moving to CC 2022 I am having major issues with saving a jpeg file! More specifically I can't get the exported file size under 6Mb!

I have followed a secific way to prepare images for use on a website for years. Specifically:

1. I create a new document the size and specs I want ALL the images to become. In this example its an 800 x 1200 Pixel dimension. 72 PPI

(normall this size image would be about 500-800 Kb)

2. I take another image (which is a different ppi and much larger - for this example lets say its 4000 x 3000 pixels 300PPI) and drop it into my new document as detailed above.

Now historically this would reformat the image I dropped into the spec of the underlyiing main document i.e 800 x 1200 72 ppi) which it seems to do

3. I export this file and save it as a jpeg (expecting it to be 500-800Kb) only to see it's 6.3 Mb!!!

And it doesn't matter what I do I can't get it smaller. 

WHY!!!???

Clearly this can't be used on a website! It's too big.

So, has something changed in CC 2022? Is there a box I need to check somewhere that has become unchecked since the update that means when I drop an image onto a document that it doesn't format it to the base document paramaters?

Here are some screen shots showing the base document info

Here is the data when I drop a lrger image onto this document and then try to save this as a jpeg showing a HUGE 6.4M

And even when I slide the quality slider to 0 it still shows as a 6.0M doc!!!

Something is clearly wrong here.

I am really hoping someone out there knows what's up coz I'm litterally pulling my hair out on this.

Hope you can help

P