My 3 1/2 month olds milk quantity has dropped by 70/100 ml a day. Also he doesn't take big feeds like before now a days.. Is it normal?

yes Hiren it is normal, because around this time your baby will be taking more amount of milk at one go and then have the next feed after a longer period of time. do check the diaper or the number of times your baby passes urine. if you think there is a marked reduction in the same, speak to the doc about it