I don't use the term because I don't want to make someone feel shame about their body but I have always read uses of 'small dick energy' as insulting, not organ size, but the culture of toxic masculinity that believes the ridiculous idea that genital size is indicative of value.
And then overcompensates with other toxic masculine traits in order to PROVE stereotypical masculinity through other means. Depending on who is wielding it, it seemed a critique of toxic masculinity and traits and not actually buying into the idea that genital size = masculinity.
When it's used by feminists, I don't see it's focus on genitals as validating the toxic masculine belief that small genitals are indicative of less masculinity but a mockery of that belief system and its toxicity. I read it as mocking a kind of masculinity not a kind of body.
Because it can be read as body shaming or connecting genital size with bad behavior, I don't use it myself. I also don't use big dick energy. I don't use gendered or body-related insults. But I think the use of the term is far more nuanced than some are making it out.
I'll add that, in this particular case, Greta has dealt with some of the absolute worst dregs of toxic masculinity from a very young age. She's gotten graphic rape threats and violent fantasies shared with her from the time she was a very young teenager.
Often from people connected to the oil and gas industry and very explicitly connecting their ability to drive gas guzzling cars and trucks with their virility, masculinity and violent fantasies. This merging of toxic masculinity and car culture isn't new for her.

@ahreaume I completely agree with you. You are absolutely 💯 spot on. We are dealing with a misogynist here. Shame on him too for dealing with a very young woman in this manner.

#ClimateJustice #Greta

@eirliani I just remember when she was 17 and this story broke. This toxic value system of masculinity and cars/oil has brought so much grossness her way. Tate yesterday was joking about raping her essentially too yesterday. The toxically masculine idea that cars/oil are key to masculinity fuels all this. Her response was contextual imo. https://www.huffpost.com/archive/ca/entry/greta-thunberg-alberta-oil_ca_5e58175fc5b60102210e12ff/amp
Disgusting Sticker Of 'Greta Thunberg' Linked To Alberta Oil Company Shocks Canadians

“It blows my mind anyone would think it’s funny.”

HuffPost
@ahreaume @eirliani this feels exactly right to me. And while her response was probably just an in-the-moment, not thoroughly analyzed thing, I’m sure on some level she was aware of this, given the precise types of abuse she tends to take.