I'll agree to disagree on 511 because it's genuinely my favorite tower from that era, and that includes the plaza where I spent many hours in Bookland and Deering Ice Cream. I do think it looked better with the original Maine Savings "ONE" signage on the mechanical cover.
Beyond that, the "human scale" (i.e., shorter buildings) argument was very powerful in Portland for a very long time. Couple that with the overall goal of being the financial and legal center for a state of roughly a million mostly poor people and the extra costs of building higher, and the fact that many of these early towers were built on or near the site of the respective banks' existing headquarters, and the way it evolved makes sense.
511 Congress: Maine Savings, earlier office survives largely intact on Casco St.
1 Monument Square: Casco Bank, HQ was on Exchange (signage still visible I believe)
2 Monument Square: I believe Maine Guaranty moved in here, on the site of their former HQ, after it was built
The one with the post office next to 201 Federal: Modernized HQ for Maine National Bank, original building next door on Exchange
Canal Plaza: Canal Bank, which was purchased by Depositors Trust (HQ in the building where Enterprise Car Rental is now near Marginal Way & Forest Ave.), shortly before they in turn were bought by Key Bank. Their former HQ is across Middle St., signage still visible.
Portland Savings made a big deal out of NOT spending money on a fancy new building in that era even as they adopted new marketing dress as "THE BANK... Portland Savings." They wouldn't move out of the Time & Temp building until the 80's, as Peoples Heritage, when they went to One Portland Square.