The fact that your babies are fraternal twins can explain the difference in their sizes. Regarding Evan's weight, your comment that he is growing in proportion is very important, which means his weight for length would average around the middle of a growth curve. If Jillian is almost as tall as Evan, she must look very thin to you, but she may always be that way. As long as she follows a consistent growth pattern there is no cause for concern.
Evan is eating more than Jillian because he is 33 percent heavier than she is, so he requires more calories. What's important is that you are providing them nourishing, healthy foods. Your pediatrician will continue to monitor the twins' growth and let you know if a problem is developing. Meanwhile, keep up the good work.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.