Ffmpeg: Image overlay on video without quality loss?

Question

fmpeg

Good afternoon, I'm trying to solve a problem - you need to overlay an image on a video using ffmpeg, I sort of figured it out, but then I realized that not everything is as good as it seemed initially, namely, after the conversion, the video size (mb) becomes 3 times smaller and the video is clearly a little darker , I tried to solve the problem on my own for several days, but I realized that I couldn’t do without the ffmpeg guru)
So, I use this command:

ffmpeg -i videoin.mp4  -i  background1.png -filter_complex "[0:v][1:v] overlay=1:1:enable='between(t,1,30)'" output.mp4

At the output I get a video with a physical size 3 times smaller and a little darker, please help me solve this problem) Thank you)