Not every boost pedal adds compression, but compression is a pretty common feature of any amplifying effect (boost/drive/dirt/fuzz)

Anything that drives will eventually squish. (I think), but is it a bad thing? I like it when my boost compresses into my overdrive, it gives it such a saturated sound, sustains for miles... My experience.

It depends on how you use it. If you're using it into a clean amp with plenty of headroom, it'll boost volume. If you use it into an already dirty amp (or in front of another dirt pedal), it'll boost the drive gain, but probably not add much volume, so basically compress the sound.

As Jayhell says, that's not necessarily a bad thing. One of the biggest reasons we like valve amps is for the musical way they compress the guitar sound.