Flight is waste, and extra weight matters
Coarse rule of thumb, by deciding to fly, total fuel weight divided by total pax weight is attributable to you*. That's a lot - it's your own weight in fuel, and probably quite a bit more.
As far as possessions, this isn't a train. Every gram of additional weight adds to induced drag which adds to fuel burn. This isn't quite as bad, but still, on a 10,000km flight, every 1 gram of possessions puts roughly 1 gram of CO2 into the air.**
If you bring a 20g metal fork, you just addded 20g of CO2 into the atmosphere. That's worse than a 3g plastic fork and 3g of CO2 (the fork is also made of 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 3g of CO2 if it's buried, or 12g 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, since it can't be heated twice (based on how aircraft galleys work; they're ovens not microwaves). Anyway, due to biocontrols it'll probably be destroyed at the destination.
Mind you, it also had to be grown in the first place, so you have all the CO2 load of factory farming to boot.
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 make 1g of CO2 for a 1g ziploc bag, than 100g CO2 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. It's not as simple as "Disposable packaging bad", and it's a mistake to think "nothing is being done". 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.
Speaking of that: Luggage. Again, since your "stuff" takes fuel burn, make your luggage as light as possible. Don't bring consumables 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. Every pound of you that you leave at the gym is a pound of fuel not burned. Also hit the airport bathroom before the flight; waste is not ejected; it is stored in holding tanks.
Because of the "tyranny of the rocket equation" - airplanes get better fuel economy as their tanks empty out. Their fuel economy is worst at takeoff when they are heavy with fuel - in fact they are unable to climb to the most efficient high altitudes until they burn off some fuel. Longer flights have worse fuel economy than shorter ones. And yes, it matters, because they don't "top up", they depart with only the fuel they need.
The gory math of whether "muliple short hops" is better (taxi, takeoff and climb-out costs a lot of fuel also, so maybe not), or whether straight-thru flight is better or worse than electric HSR - is too much for this answer, but it's a place to look.
A sidebar: Waste-stream recycling is a thing
Recycling from intercontinental jetliners is weird, because of biocontrols. But to speak of recycling generally -- 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. Likely so if your destination is an eco-minded place like western Europe or the usual coastal ports of entry in the US. So the recycling may be happening even if it is not apparent!
* You decided to fly, and that makes you responsible for your share of the airplane's weight, the fuel, and the fuel to fly the fuel. Just to throw a number out: An A380 carries 81,890 gallons of fuel. Say it's super-tight all-economy, seating 819 passengers (makes the math easy). Each passenger burns 100 gallons of fuel. Fuel weight 6.8 lbs/gal. so 680 pounds of gas. That's worst-case, but on a rather efficient bird. So I am very comfortable with "your own weight in fuel". However since possessions and bodymass are incremental, we look at those differently.
** Based on a comment that every extra kg of payload requires 0.2l of fuel (nearly 0.2kg) for a 6000km flight, and 1g of fuel makes 3g of CO2, so 0.6kg. I am stretching that to a 10,000km flight and it rounds to 1kg/1kg. Mind you Jet A is about 15% lighter than 1kg/1l, except the tyranny of the rocket equation makes the fuel burn probably 15% worse, so I'm calling that a wash.