Actors for Hermann

An absolutely fascinating series. Have just started Heimat 2. I have been wondering why in some instances Reitz uses a different actor when a character gets older, but has Maria played by the same 34-year old actress the entire Heimat series, from age 20 to age 82. I did not think there was any need to replace the soldier Anton, for instance. Hermann is even more confusing: OK, the actor who played him in 'Hermannchen' was obviously very boyish, but his successor in the last two instalments that same series gives off a totally different 'vibe' -- too short, for one. However, the Hermann in Heimat 2 is a perfect follow-up of the boy in "Hermannchen'. I'm sure there must be a rationale behind this, but what??