The Lightroom one was processed from raw. Also, by brightening it a lot, the noisy high-ISO grain becomes more apparent. Noise is famously incompressible, so it leads to a much larger file size.
Also, pulling more data from your 14 bit or 16 bit raws results in more noise in the end compared to the straight-out-of-camera 8 bit JPEGs.
For example, if you render Gaussian noise in photopea and export as JPEG 100% quality, it has 9.2MB. If you reduce the exposure by -2 it goes down to 7.8MB. That's partially because more parts of the noise are effectively black pixels, but also I believe because of the earlier mentioned effect.