They sound very small, ours have just gone at 24 weeks of age and killed out at around 60kilo and their live weight was 90kilo, I would measure them again using this method below and see what you come up with, we used this method and your mesurments and it doesnt seem to add up correctly.
What feed ration are they on, how many times a day do they get fed? were they ill as weaners?
They should be eating at least 5lb a day each, split in to two rations 12hrs apart for maximum weight gain. If you increase your food and they get fatty dont worry too much, strip there ration to half 2 weeks prior to slaughter and there fat will dramaticly decrease.
IMPERIAL - Weight of your pig in POUNDS
1:Obtain a fabric measuring tape or a piece of string to use as a measure. If using string mark the dimensions on the string and
2:then measure the dimensions using a steel tape measure.
3: Place the tape/string under the pig just behind the front legs and measure the circumference of the pigs girth in inches. This measurement is known as the Heart Girth.
4:Then measure the Length of the pig along its back from the base of its ears to the base of its tail, again in inches.
5:To calculate the pigs weight, first square the Heart Girth to get the Girth Result.
6:Now Multiply the Girth Result by the Length and DIVIDE by 400.
7:You now have the weight of your pig in Pounds.
Hope this helps