Flight is waste, and extra weight matters
Coarse rule of thumb, you'll burn your own weight in jet fuel, along with the weight of your possessions. Unlike trains, your decision matters -- with airplanes, every gram of additional weight adds to induced drag which adds to fuel burn. Every gram of fuel burned makes roughly 3 times its weight in CO2.
Oh, you bring a 20g metal fork? Then the airplane burns an extra 20g of jet fuel making 60g of CO2. That's worse than a 3g plastic fork and 3g in jet fuel (they're both made of ancient petroleum). And they won't know not to load a packaged meal for you, so they'll load it on the airplane anyway; that means the weight of your own meal is added to the fuel burn that will happen in any case.
Burning petroleum is worse than making packaging out of it and burying it in a landfill. Landfill is a kind of carbon capture, though certainly not a preferable one. When you landfill a petroleum product, it's a wash - it doesn't convert to CO2 since it doesn't burn. So that 3g fork makes 9g of CO2 if it's buried, or 18g if it's incinerated.
Also, refusing the meal is unlikely to result in any environmental savings. The logic is "they'll save it and serve it to someone else on a future flight" - not likely if it's heated/prepared. Anyway, due to biocontrols it'll probably be destroyed at the destination. You might as well just eat it.
Mind you, it had to be grown in the first place.
So you're actually doing worse by bringing your own food. (unless that's dietarily important; yet another factor to balance.)
If you do bring your own food, focus on lightweight packaging even if it is throwaway. My "go-to" is Ziploc bags. Better to do 1g fuel-burn for a 1g ziploc bag, than 100g fuel burn for 100g of quality tupperware. Twice since I assume you'll bring it back.
Look at it a different way
Keep in mind, Big Minds are already working on the environmental-waste-of-aviation problem. "Disposable packaging bad" is an eco-novice's knee-jerk, and it's a mistake to think "nothing is being done, and I am the only hero of the environment". It's more complicated and subtle than that, especially when you start thinking of the damage of poor biocontrols, e.g. disease, invasive species, that kind of thing. They sterilize foreign waste for a reason.
Speaking of that: Luggage. Again, since your "stuff" takes fuel burn, make your luggage as light as possible. Don't bring a bunch of stupid things that are readily available at your destination. That's another "balance of priorities": re-buy something you already own, or spend 3x its weight in CO2 bringing it with you.
Speaking of that: You. Not to nag about your weight, but every pound of you that you leave at the gym is a pound of fuel not burned.
A sidebar: Recycling doesn't work like you think
Recycling from intercontinental jetliners is weird, because of biocontrols. But to speak of recycling generally -- again, you're not the solitary hero. Recycling efforts are already made on waste streams. Metals are separated by magnets or eddy currents; and laborers pick out cardboard, sacks of newspapers and grabbable plastics. Many cities have found it's cheaper to have one waste stream and have machines/laborers separate recycling, than to have citizens have multiple waste cans.
Cities keep the blue bins due to citizen pressure/guilt, but it still needs to be picked through because people put lots of stupid stuff in the recycle bins. The upshot is, the "blue bins" are often "recycling theater", and actual recycling occurs farther down the waste stream, whether there are blue bins or not! Therefore, don't presume that your flight waste isn't recycled or handled responsibly. It probably is - if your destination is an eco-minded place like western Europe of the usual coastal ports of entry in the US.
Recognize that many "eco" gestures are empty
As you see, it's really easy to do easy, simple things that really feel like you're being 'eco' but don't do much at all. Don't be distracted by stuff that seems "easy" merely because you have more control over it. Look behind the curtain and see what really matters, and focus on that. And make the hard calls: "Do I really need to pack that?" "Is this trip necessary?"