My surgeon explained that they cannot 'see' the cancer during surgery - it just looks like the rest of the breast. So, if I hadn't had an MRI scan, he could have performed a lumpectomy removing the 10mm of tissue around the DCIS microcalcifications which showed on my mamogram and thought he'd got it all. The MRI showed the IDC was 35mm, expanding well beyond the DCIS, but not everyone is offered/can afford ($700 out of pocket) an MRI prior to surgery. Without it, my experience would probably have been exactly like yours. Small lumpectomy revealing much bigger surgery needed.
I'd ask on here for recommendations for a plastic surgeon specialising in reconstruction and talk to them. They will know all the ins and outs of the risks to implants from radiotherapy. You may even find that you lose enough breast tissue that you'll want a slighter larger implant on that side after everything, and they can talk you through how all that can work.
I was very happy with both my Melbourne based surgeon's (breast and plastic - I had a mastectomy with immediate implant recon) if that is where you are based.