I can totally relate. My second child, DD, now 9 months old has always been drinking less formula than the average baby. I BF DD for 6 weeks, but never felt full or engorged like I did with my older child. Due to latch problems, I switched her to formula and noticed that she usually drank about 3 oz per feeding. I started charting her feedings and realized that she usually averaged about 17-20 ounces around 2-4 months old. I fed her as often as 9 times a day thinking she would drink more, but it didn't matter because she would drink no more than 20 ounces a day. I brought this up to her pediatrician several times. The doctor asked if I was preparing the formula correctly (i.e. level scoop vs. rounded scoop) because the amount seemed low, so maybe her bottles were mistakenly more concentrated. At one point she was even in the 75th percentile (she was 6 lbs 6 oz at birth, so not exactly big). Anyway, because she was gaining weight and growing the doctor chalked it up to her having a slow metabolism.
Now at 9 months, her intake hasn't really increased. We just switched to 4 bottles a day. I put 6 ounces in each, but she doesn't always finish. So in a typical day, she gets about 20-24 ounces and 3 solid meals. She goes to daycare full time and is ALWAYS sick. On those days, her intake can be as low as 17 ounces with no solids. I still record her intake at every feeding (yes I'm totally obsessed, but learning to relax a bit more). I started tracking because compared to my older DS, she is taking a lot less. DS was more the norm and usually drank 5-6 oz per feeding around 3 months of age.