Discrepancy in Image Size After Converting HEIC to JPEG

I encountered an issue while downloading photos in different formats and would appreciate some insights.

I captured two photos: one in the default high-efficiency format (HEIC) and the other in JPG (by changing the setting on my iPhone). When I downloaded the unmodified versions of both photos, they both had identical dimensions: 4284 x 5712 pixels.

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However, when I downloaded the same files in the "most compatible" JPEG format, I noticed a significant discrepancy. The photo originally taken in JPEG format retained its original dimensions of 4284 x 5712 pixels. On the other hand, the HEIC photo, upon conversion to JPEG, experienced a drastic reduction in size (>50%) - 1536 x 2048.

I understand that there may be some loss in quality during the conversion process, but it's perplexing why the HEIC photo's size would be reduced by over 50%. Could someone shed some light on this issue?

Thank you for your assistance.

Luka