The biggest name in men’s diaper bags is certainly Diaper Dude. Their Messenger I was one of the first-to-market diaper bags for dads. My first bag was the Messenger I in grey with orange zippers. It’s a very nice bag that I use for quick trips. It has a nice form factor and plenty of pockets. In terms of storage, it’s small to medium sized. When out for extended periods or if you have a baby that goes through diapers as fast as my little guy, you might need more space.
For a ton more storage and amazing build quality, check out the Large Sling from WaterField Designs. I’ve been using their laptop products for years and love the amazing customer service, quality, and the fact that they are made in San Francisco, right here in the USA. The owner, Gary, is a great guy that I’ve exchanged a few emails with way back when they were a tiny shop. This bag is extremely spacious, but is so soft and supple, it conforms to the shape of your body perfectly. The bag is machine washable, which is a great bonus, for those spills and diaper explosions us new parents are all too familiar with.
EDIT – Sadly,The Large Sling is no longer available from SF Bags