It's all up to you and your personal definition of "portability".
My current work laptop is nearly identical in size to the old pre-Retina 15" MacBook Pro. I don't find it problematic.
Previously, I used a 2006 15" MacBook Pro as my primary laptop for many years.
Prior to THAT, I had 12" PowerBook G4, so I was used to "small". Prior to THAT I used an ultra-portable PC (Sony PictureBook) that made even the 12" PowerBook look big.
That said, it also depends on what amount of computing power you need to carry with you.
If your workload is such that you see a large improvement in productivity using a quad-core CPU and especially a discrete GPU, then yes, the 15" is absolutely worth it.
If a dual-core CPU and integrated graphics are fine for your workflow, and you want maximum portability, then a 15" probably would be "more than you need."
In my case, when I got my 15" MacBook Pro in 2006, I made the choice to move to using just a laptop, with no desktop computer. (I'd "dock" my MBP, plugging it in to a larger external display, keyboard, mouse, and external hard drive.) With the one computer as my only primary-use computer, getting the larger/full-feature MBP made perfect sense.
Since then, I have gone back to using a desktop as my primary home computer, and don't need much in the way of portable computing power (I actually use an old 15" Titanium PowerBook G4 as my primary mobile computer now, largely out of nostalgia.) Whenever I decide to get a new portable, I'll probably go for the new 12" MacBook. It provides "enough" power for my portable computing needs, while being ultra-small-and-light.