I don't personally have a problem with Zukki's weight, but the wota are right about two things:
1) As an idol, it's a part of her job to take care of her looks, including her weight.
It's a job that is strongly based on looks, kind of like being a model... So if it appears that she's not even trying to improve her "job performance", some fans will think she's undeserving, lazy, etc. I think some fans trying to come up with excuses for her like having a disease or being over-stressed are sort of making things worse because they're making it seem as if Zukki has no control over it, taking away her power to change her own body. Sure, there may be some underlying issues and she might have to work out more than some other members to achieve a similar appearance, but most likely she's just not watching what she eats and not exercising enough - the simplest answer is usually the correct one.
2) No matter how cute she is now, her looks would only improve if she lost some weight (in a healthy way, of course).
I think fans who are in this fandom to see the girls improve in every way and realize their best possible idol potential are also frustrated that this girl is stuck in her current weight, kind of like being stuck with one hairstyle (which Zukki is, too). I'm hoping that she'll pull a Makoto who went from being chubby to looking energized and fit. She was always a genki dancer but her slimmer frame really brought it out, made it impossible to miss. Zukki has very lovely facial features (big eyes, lush lips, great smile) so her popularity might significantly increase if she managed to do the same as Makochan. Not even just because of her improved looks, but fans would think "look how hard she worked, she must be so passionate, I will definitely support her from now".
And yeah, Japan weight context =/= American & European weight context. It's always shocking to go to Japan for a while and then return back home.
I also think that making fun of a girl or woman for her weight is a low thing to do. It's one thing to attempt to discuss a topic in a rational way (for example "what it means to be a chubby idol in Japan") and another to start throwing around insults and essentially body-shaming her.
Since Zukki has been this way for quite a while now, it appears that she's happy being in this comfort zone of her current weight and also relative unpopularity. It's great if she's happy, but it's also boring if she's not fighting to survive. It becomes more than just a weight issue, also visible in the way her personality has quieted down. In the entertainment business, a person should never get comfortable because it takes a lot of work to keep people's interest up.
Like I said, I don't have a problem with her weight, I have more of a problem with her having become quite boring recently.
And on a completely different note, clearly J-wota who think that all foreign fans are just illegal downloaders are simply wrong...