For me this song is about some dude that is very conflicted. I think that the general melody of the song describes a guy who is sad and doesn't have many things sorted out.
then this guy meets a girl that he thinks (or others tell him) should be good for him, but he has a ver dull way of approaching this situations in general, almost like it is a duty (You're my size I need to try you on ). he starts to know more this girl but doesn't form a complete opinion about her (someone in between the right and wrong), but despite him being melancholic and gloomy she doesn't give up on him (through everything you kept your wings apart (she keeps 'flying' with him)) and doesn't change the way he treats him or feels about him (trough everything you stayed the same old heart).
then he starts realising that when he thinks he cant be happy and that there are no such emotions inside him, she starts bringing some joy out of him (and when i've had enough, she drains me when im empty) and she doesn't mind that all the effort is made by her and that its up to her too make this work (she fills herself she takes it all), she takes every bit of happiness she can get from him. he thinks that he is committing way too much with her, but at the same time some of her personality is being transmitted to him (in too deep, she's spilling over me) and he starts to accept this (go on and have it all).
He then starts panicking, he thinks that they are way to different to be together, she is way to cheerful and happy for him, so he tries to look for a way out (Running through this maze you hold me in, searching all these days to find the end , your everything, your everything I'm not) but this collides with the part of him that enjoys this connection and that would do anything to stay together (but im anything im anyone you want).
After the second chorus he reinforce the idea of this conflict within him, he doesn't know what to do (Your everything you're everything I'm not, I'm anything I'm anyone you want ). in the song dave screams this quite strongly which emphasises the struggle.
finally, after the last chorus, the song goes into this hard, dark outro, he hasn't made up his mind yet and you can tell that he is having a hard time deciding. and at the end the song ends abruptly, leaving us wondering what his desition was.
For me this song is about some dude that is very conflicted. I think that the general melody of the song describes a guy who is sad and doesn't have many things sorted out.