I made clips with my phone and created a video using the HD1080p 29.97 fps profile, and included zoomed and blurred copies to make them fill the empty space, as shown below.
The bitrate may vary in both exports, right click on the exported video, go to properties, then see what are the video bitrate. If one is larger than other, it means that the bitrate is increasing size.
Most of the times bitrate is the culprit, I even forget sometimes to change the bitrate, and it gives me 1GB instead of 100mb. Usually changing the bitrate is the best practice I do to save my storage.
Large areas of blur compress more easily. If you had used solid color background it would compress even more. With the vertical the entire video surface is full of detail.