6 mb inputs => 45 second video => 84 gb estimate file size => wtf?

I'm using premiere pro to mock up an explainer video I'd like to have someone else make for my company.

I recorded a 45 second explanation audio track (1.1 mb .m4a file)

I dropped some background music in (2.25 mb mp3 file)

Added 10 pictures showing my ideas for visual content (png files ranging from 100 kb to 300 kb totaling approx 2 mb)

I go to export with h.264 / youtube 1080 preset and get an 84 gb expected file size and a ridiculous encoding time.

I've tried other options like "match source - high bitrate" (vbr 1 pass, 10 mbps target, 12 mbps max) and that drops it to 53 gb but that's still pretty ridiculous.

I'm struggling to understand when all my input files total approx 6 mb how the output file size can be approx 10,000x as large.

Any help would be appreciated.  Thanks in advance!