Observe your foot all by itself...no shoe. Look at the distance from the corner of the heel to the inside corner of the ankle--where the shin meets the top of the instep.
It is that distance, at a minimum, times two, that has to go into the top of the boot. The elastic will allow the boot to open up some but if the boot is designed to fit close to the shin, no amount of elastic will be sufficient to allow the foot to pass through.
How close the top of the boot fits the lower leg may indeed vary with the manufacturer and how willing they are to over-stress the elastic.
Bottom line is if the foot goes into the boot without breaking threads and/or strands of rubber when entering, and the foot feel fine, and the boot closes back up such that there is no significant slippage in the heel, it's probably as good as it gets.