OpticalLimits

Small backpack (25x20cm inner)

stoppingdown

I own two LowePro backpacks for trips — they contain all my gear, with the exception of specialty lenses (vintage stuff with adapters), that is 99% of my usual activity. I'm perfectly fine with them.
Now I need to increment short walks near home because I need to lose extra weight 🙂 and I need a very small backpack to carry along just a couple of essential zooms (18-50mm + 70-200mm) plus bodies. In terms of size, 25x_20cm (inner) would be fine. Searching on Amazon and other places I'm finding lots of good and cheap stuff, but considerably larger.
My only strict requirements other than size are weight (but should come by itself), decent quality (i.e. don't break after a few years) and rain protection.

Suggestions welcome.

mike

I've always like the quality of the Think Tank bags. The Streetwalker 2 seems to be just the size you're looking for:

https://www.bhphotovideo.com/c/product/1340538-REG/think_tank_photo_475_streetwalker_v2_0_backpack.html/specs

They have a couple of Slings which may work for you.

toni-a

Well I have case logic backpack since 2016 and it's still in perfect condition, it's water proof and had proected gear from damage after falling so many times.
I also have since 2007 Canon Backpack and it's still pretty awesome, Lowepro cost twice as much with identical quality.
Practically I have EOSRP plus 3 lenses in case logic backpack
https://www.caselogic.com/en-us/camera-bags/camera-backpacks/case-logic-slr-camera-backpack-_-caselogic_85854221481
My unused gear is in a large lowepro back pack I carry very occasionally when I need 2 camera bodies
so my suggestion would be either the case logic or Canon backpack like the excellent 200EG but it seems discontinued, for your need I think this one should do
https://www.adorama.com/ica100s.html
Strange enough Canon backpacks are excellent, a great value for money, and usually cheaper than competitors !!!
If you have no problem with the name Canon on the backpack I would recommed going Canon !!

me

Recently I purchased this one here - and I like it quite a bit:

https://www.thinktankphoto.com/products/photocross-13

There's also a smaller version of this already small (sling-)backpack

https://www.thinktankphoto.com/products/photocross-10

stoppingdown

I've looked at the models, but they are too large:

Think Tank Photo StreetWalker V2.0: 44 cm
Case Logic camera backpack: 43cm
Canon Sling Backpack 100S: 35cm
PhotoCross™ 10: 45cm

Just to have a reference: a good fit would be the Lowepro Micro Trekker 100, whose exterior max. size is 26.5 cm:

https://www.amazon.com/Lowepro-Micro-Trekker-100-Camera/dp/B00004TX7A

It has actually been my first backpack, but the copy I have is more than 20 years old, it's still in very good conditions but not great, and unlike other similar products by LowePro doesn't have a specific rain protection — so I have demoted it for use at home only, as a storage of items that I don't carry during a trip.