RBC Capital’s Mike Abramsky, whose estimate that Verizon (VZ) had fewer than 100,000 Apple (AAPL) iPhones in stock for pre-sale last week seemed low, has issued a new report with some more aggressive numbers.

He now estimates that Verizon will sell more than 1 million iPhones in its first week and 3-4 million before the end of March. “This is strong,” he writes, but it’s less than the 5 million iPhone 4s AT&T (T) activated in its first full quarter of sales last summer. He offers seven reasons why this might be.

• The iPhone 4 at Verizon is new, but not novel (vs. when AT&T first launched its version of the iPhone 4).
• A lot of existing Verizon customers with Google (GOOG) Android phones are still on contract.
• AT&T offered customers an early upgrade option; Verizon did not.
• There are a lot of AT&T iPhone owners who might like to switch, but are still on contract.
• Verizon’s data plan ($30/mos.) is more expensive than AT&T’s $25 and $15 offerings.
• AT&T’s first-quarter iPhone 4 sales covered 13 weeks vs. Verizon’s 7 weeks.
• Some potential Verizon customers are holding out for the iPhone 5, expected to arrive this summer.

Also on Fortune.com:

• Verizon iPhone: The reviews are in
• Rate plans: How AT&T, Verizon, Sprint and T-Mobile stack up
• Survey: 44% of Verizon Android users likely to switch to iPhone on Day One

[Follow Philip Elmer-DeWitt on Twitter @philiped]