There are many machlokes Rishonim involved: why Maror doesnt require haadama (either because it is patared by earlier haadama or because it is after hammotzi), theres a machlokes whether to make borei nafoshes immediately after eating a kezayis karpas since there is a long hefsek.
By avoiding a kezayis karpas you avoid this machlokes since you certainly wouldn’t make a bracha achrona. This is how mechaber paskens.
(though it is impossible to satisfy EVERY shitah since for example the Rambam paskens that karpas requires a kezayis like every achila, Bach says to be machmir like Ramabam)