Video size reduced after adding audio and exporting

I add a video of size 950mb and 1920 x 1080 p resolution and 30 frames/sec. When i add audio to this and export it turns out to be of much smaller size i.e. 725 mb and even lags a bit. Why does this happen ? Every time I make edit to this and export it again it the result will be a much smaller file with low quality and lag. How to prevent this ?
These are the setting I use while exporting -
Resolution - 1920 x1080
Aspect ratio - 16x9
Frames - 30/sec
Qualilty - 67 %
I also set the Settings>Video> 1080 p 30 fps

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