It's puzzling how the size of Microsoft's Defender full definition db doesn't grow continuously, considering the number of virus signatures grows should grow over time. There's a limit to what can be gained from improved compression or definition formats.
Recently there was a strange big drop:
2018/09 database: 200 MB
2018/12 (I think it was): 160 MB
2019/03: 80 MB
2019/04: 120 MB
If they regularly prune older viruses, doesn't that defeat the idea?