Yes you could increase the rotor size by using an adapter.
The Shimano SM-MA-R160D/D adapter is what you'd need.
Here's a couple of diagrams and a webpage that could be useful to you......
https://si.shimano.com/api/publish/s...7070-4328C.pdf
https://si.shimano.com/pdfs/si/SI-8NU0A-001-00-ENG.pdf
https://si.shimano.com/
....Felt VR5 which looks to have the 105 disc rotor mounted directly to the frame
The rotor is attached to the hub, the disc brake caliper is attached to the frame.