You are exporting different file sizes, even when selecting all the same JPG settings, for two reasons: the image content is different (and hence more, or less, compressible into JPG at given output settings) the quantity of attached metadata (informational, and a record of the LR edits) is different The first reason is far more significant size-wise. The second presumes that full LR metadata is being included - which you can limit. Pixels must be different if both versions of a photo have undergone differing ISO / WB / demosaicing and subsequent processing - despite both deriving ultimately from identical camera sensor output. Most important for file size are noise reduction / smoothing / artificial grain - sharpening of various sorts - tone and hue adjustments leading to blacks and whites "clipping", etc. You may have cloned or otherwise locally edited too. More basically: what's even SEEN IN each image version and, how that's duly rescaled for output, may differ due to cropping / lens or perspective corrections, etc.
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