Adding small image increases file export size dramatically

Hello - I am VERY new to PP 2020. As in 2 days new, so apologies if this is simple and I'm missing something, but I have searched and scoured for over half a day for help on this issue and cannot find assistance. I'm on Mac Mojave 10.14.6.

I am working on a project with an hour-long video that I have cleaned up and exported using H.264 format to .mp4 with the "Match Source - Adptive High Bitrate" preset selected; no other export changes.  The file is fine and size is 451Mb - perfect.  I realized after export that there's about 1-2 seconds of empty space (black screen) at the beginning, so I created a still "bumper" image in Photoshop, same size as the video (1920x1080 at 72ppi), making for a small 380Kb image.  I imported that image to the project, dragged it to the video layer, created a default "crossfade" transition and exported again.  The file size upon export, using exactly the same settings, is now 6.8Gb.  Yes, GIGAbites.  From 451 MEGAbites.  Just by adding a 380Kb image?? How can this even be possible?  A 6.8Gb file is way too big to use.  Can anyone please help me understand why this is happening and what to do about it?  It just makes zero sense that the file size would increase by almost 16x by adding a tiny static image with a fade.  What if I needed to add dozens of images?  I'm so frustrated!  Literally half a day trying to find answers and I've found none, so I praying someone here can help.  I've attached screenshots of the export dialog box without the front bumper image and with.  I also tried to export a frame as the image and use that in the front the same way, also with the same results. I have no clue why the file would be so exponentially bigger? Thank you in advance.