Photoshop Save for Web Jpeg size

Can somebody shine some light for me on how PhotoShop JPEG Save For Web works?

Here is my problem:

1. I start with original.bmp (32.1MB on hard disk).
2. Then I run File | Save For Web... , using following options:
   
   1. Optimized (Checked)
   2. Embed Color Profile (Checked)
   3. Convert to sRGB (Checked)
   4. Quality = 60

What I get is original.jpg of 474KB.

Next I add white rectangle on top of the picture, increasing picture size by 200px.

This increases size of bmp to 33.7MB from 32.1MB. This makes sense to me as I have increased picture size by 200px. Here is how I did my math for sanity check:

Original picture dimensions: 4134x2717
Original picture pixels: 4134x2717=11,232,078

Increased picture dimensions: 4347x2717 (added 200px vertically - white rectangle)
Increased picture pixels: 4347x2717=11,810,799

11,232,078 / 11,810,799 = 0.95100069

This 5% increase matches the increase of size of bmp image on file system 32.1MB/33.7MB= 0.95252226

So far so, good and this makes sense.

The issue I have is when I try to "Save for Web" new bmp picture with increased dimensions (added white rectangle), I get 604KB jpeg picture. What I get is: 474/604=0.78 or roughly 22% picture increase.

Considering that I have added just white rectangle and jpeg format has compression, shouldn't I get 5% or less files size increase? Can some explain to me how this works or if I am doing something wrong?