I believe you have to show that the detriment to your child not attending the school would be more significant than the impact on the other children being in a larger sized class (or something to that effect)
For Reception, that is for cases where adding an extra child to a year group would mean taking it over PAN, but no class size in Reception, or in year 1 or year 2 as that intake moves up through the school, would need to go over 30 as a result. e.g. a PAN of 25 would normally see all three years being taught as separate classes of 25.
However, e.g. a PAN of 45 would normally have Reception taught as 22 and 23, but then years 1 and 2 taught as a block of 90, 3x30, with either one or all of the classes being mixed yr1 and yr2. Therefore, an appeal for Reception would be heard under Infant Class Size legislation because although the immediate effect of one extra wouldn't be to take a class to over 30, there woukd be no way to keep classes in yrs 1 and 2 for that intake to 30 as the year group progressed.
Also, the bar for 'unreasonableness' is high and applies to the process (e.g. the application was ranked other than according to the published oversubscription criteria for a school / the LA ignored the rules for allocating places etc), not to the outcome - so the process being followed completely correctly but the result is that the parent has DC in different schools isn't legally unreasonable.
That being said, occasionally parents appealing against an Infant Class Size refusal do get lucky.