Why isn't it the other way around? This seems to cause the abdominals to stick out rather than flatten
By breathing out as you drop your legs or lower weights you reduce the pressure on the rectal abdominal muscules. This also will allow the muscules to stretch after crunching them. It will help them to get stronger. As for them sticking out when you breath out it is actual the reverse. When you lift you legs you are crunching the muscule. That is what makes it stick out.Because your back is flush to the mat is will feel as if your muscules are not pushing out. Now when you drop your legs you may feel like the muscules are pushing out but it is because you most likey have your back arched too much. As this happens the muscules will protrube somewhat but if you notice or have someone look you will find that the are elongated not compacted.
This is a common issue with weight lifters. Holding your breath may help get that last rep out but it can push out as well. If you have this happen you will know it. The only thing that can be done is to not stress them again and let the body fix itself naturaly. If it is too bad you may have to see the doctor.