So I figured out that despite the fact that only the composer is calling render, the original renderer also needs to have it's size reset in whatever resize method you have.
The composer is based on the original renderer, and I had commented out the resizing of this object and only resized the composer.
This is something I must have overlooked, hope this helps others!
camera.aspect = Params.win.w / Params.win.h;
camera.updateProjectionMatrix();
//MUST ALSO UPDATE RENDERER SIZE
renderer.setSize(Params.win.w, Params.win.h);
cssRenderer.setSize(Params.win.w, Params.win.h);
//composer
effectFXAA.uniforms['resolution'].value.set(1 / Params.win.w, 1 / Params.win.h);
composer.setSize(Params.win.w, Params.win.h);