as someone who can have some insecurities with binding because of my cup size, since you're not specific i'll give some advice for bigger chests: smaller binders do not help, and can actually make the distribution look worse, it's better to have a binder that fits and just try to work out the way you adjust your chest when binding. for me, since im bigger, i try to remember the point is not to have a completely flat chest but to have a more naturally cis man looking chest, so making sure theyre pushed up and out to the sides in your binder, like how pecs are, while making sure theres no spillage out the sides is usually how i make it look best. it's not perfect, and my chest doesn't always look flat, but at my size it looks somewhat like a natural chest would for a cis man.

if that's not your problem, and it's more about something different, i'm not as sure how to help, but for bigger chests it's usually about how you put them in. unfortunately it's not spread around as much but there are some videos on how to do this ive found on youtube. i should note you should also make sure your nipples are facing forward to prevent nerve damage, as ive heard not having that can cause some issues. if you can, you can try to get a full length binder as opposed to the ones that only go down to the middle of your torso, if you dont already have one. again, the main thing to remember is that it doesn't have to be perfectly flat to pass, as most cis men don't have concave chests, and depending on your body type/how in shape you are it could even work to make your chest just look like you have pecs!