I also heard people saying that SegWit is actually a block size increase. Wait what? So you can do a soft fork to increase the block size, and Bitcoin Core clients installed in 2015 and never updated will accept those blocks as valid and add those blocks to their Block Chain?
Am I understanding this correctly? Before SegWit, the Block Chain on my PC (running 2015 software) increases by 1 MB per 10 minutes on average. After SegWit, the Block Chain on my PC (running 2015 software) increases by 1 to 2MB per 10 minutes on average?
Not exactly... SegWit was a bit of a "workaround" to the block size limit. By putting a chunk of data into the Witness, you're theoretically increasing the block size
for SegWit compatible nodes. Older Legacy nodes (like your theoretical 2015 software), just ignore the Witness part, strip it off and only store the "legacy" data, so will still end up with max 1 meg blocks.
Also, as a note, the blockchain size increases by
a maximum of 1 MB per 10 minutes on average for your old node... it depends on how full the block actually is... sometimes they're empty or only have a few 100s or 1000s of bytes.