iCloud Photos Library

I have finished uploading all of my photos and videos to iCloud from my Mac roughly around 151 gbs worth. I have optimized versions turned on and when I go to check how much storage the photos application is taking up it starts off at around 1.5 gbs. Then the next couple of days I check it again and see that the photos application has taken up around 4.5gbs. And it increases even more up to around 10 gbs. I don't understand why this is happening.