Why does chromosome multiplicity lead to abnormalities?

[Uncommon Ancestor]

So I was reading this recent thread about people having multiple X chromosomes and both Klinefelter syndrome and the triple X syndrome were mentioned. While it would be apparently more obvious why XXY people might have problems (feminisation of males bearing a Y chromosome), I can't reach to understand why does having more than two X chromosomes lead to abnormalities in females.

My knowledge about the subject is pretty basic, so even though I can understand why the existence of a single one chromosome could trigger the apparition of otherwise recessive traits (such as hemophilia and daltonism) I can't figure out why in the reverse case things go wrong as well. Could it have something to do with over-expression of some genes? I don't know. That applies both to 3X syndrome and to Down's, amongst other cases, too.

Also, as far as I know, similar cases in plants tend not to cause problems at all (polyploidy), so why should it cause trouble in humans?

An explanation would be highly appreciated, thanks in advance.

Edited January 28, 2018 by Uncommon Ancestor