Ooooh, poor thing...that crop is huge! The problem is, this could be anything from sour crop to an inherited muscle disease that affects the tissues surrounding the crop. It could even be a bit of something she swallowed that is now causing an abscess. Without an xray and proper veterinary checkup, it's just too difficult to diagnose.
To the OP, I want you to catch her and smell her breath. Yes, I know this sounds weird, but just trust me. Try to gently push down on the crop and tell me if it smells normal, or has a slightly rancid smell to it. If it does, then it's sour crop; a candida infection and she needs antibiotics AND antifungals. If you don't, then it's something else that could be potentially more serious.
Also, start feeding her smaller meals consistently throughout the day rather than one or two large ones. Until the mass goes down, regardless of the cause, this will help her a lot.