At some point, I was flying a low cost airlines with just 1 ordinary backpack.
But I didn't pay for baggage, and the bag indeed exceeded the size requirements: it was well packed.
I had to hastily pay extra for the baggage, leave it behind, and then wait for it upon landing.
I hate waiting for baggage claim on arrival, and there's also a possibility that it will be lost
...and at that point it costs the same as a normal flight.
On the next trip, there was only one flight that suited me and it was a low cost one (and with the added baggage... well, it stops being low cost).
So I've read the tariff rules, and it stated the following:
I can take a 40x30x20 bag, and a laptop bag or a camera bag.
So I've packed my trusty North Face Borealis (just for reference),
But! I've packed my camera bag inside and in it I also loaded anything that shouldn't go to checked baggage:
A charger, USB cable, power adapter, toiletries, a pair of socks, a T-shirt, a camera...
Basically anything I could fit in there.
To be honest with you, I only really use this bag for this purpose only.
I apply this trick even if I don't take the camera with me, which is rare, but in this case I can stuff this bag even more.
First of all, the bag serves an organizational purpose.
Second, I can still take the bag out and send my backpack anywhere and don't lose anything critical.
Third, I can take it out before check-in desk, wear it on my shoulder.
And that isn't against the rules: I have a backpack that now fits into 40x30x20 box and a camera bag, which is allowed.
There's one downside: it's inconvenient.
Taking it out before checking, carrying it, and putting it in after. But it's fine.
It's only for a very short time, and the whole process doesn't take too long, so it's not a big deal.
In general, I dislike traveling with a suitcase, and limit myself to just one regular sized backpack, and I'm a greedy bastard.
I'll take 5 minutes of humiliation for the benefits.