I think some generalisations are worse than others. In my anecdotal experience at least, that generalisation is not true. Take for instance the USA and the democratic republic on congo. Which citizen donates more to charity on average? I believe it would be the USA.(1) I am not from either country, so i have no stake in it. But logically, if you have little to give, you can't give a lot by the very nature of not having a lot. I wouldn't expect them to. The only way to help others is to help yourself first. Also, humans are self preserving by their very nature.(2) This means that if push comes to shove, and you need the money for survival, you are more likely to use the money on yourself than on others. Money is much more of a necessity in someone who earns less compared to someone who earns more. I found a study(3) with the results below of charitable giving sorted by income level, with the bottom 90% of income earners in one group, the next 7.4% in one group($100000-$200000), $200,000 and $1 million (2.1 percent of the US population); and finally, donors with an annual household income equal to or greater than $1,000,000 or more (0.2 percent of the population).

These were the results(quoted from the paper) "Households with an annual income of less than $100,000 (“average” income households) • 23.4 percent give to combined purpose charity organizations, giving an average household gift of $330, cumulatively making up 34.8 percent ($7.7 billion) of the funds raised by combined purpose charities.

Households with an annual income between $100,000 and $200,000 (“above average” incomehouseholds) • 48.1 percent give to combined purpose charities, giving an average household gift of $554, cumulatively making up 9.8 percent ($2.16 billion) of the funds raised by combined purpose charities.

Households with an annual income between $200,000 and $1,000,000 (“wealthy”) • 62.1 percent give to combined purpose charities, giving an average household gift of $7,087, cumulatively making up 46.1 percent ($10.19 billion) of the funds raised by combined purpose charities. Households with an annual income equal to or above $1,000,000 (“very wealthy”) • 60.3 percent give to combined purpose charities, giving an average household gift of $17,072, cumulatively making up 9.3 percent ($2.06 billion) of the funds raised by combined purpose charities."

So i tried to calculate the proportion of income given by group. It's difficult as not enough data is given. Assuming everyone earning below $100000 earned exactly $100000, with an average household gift of $330(from the study), that is 0.33% of income Assuming everyone earning between $100000 - $200000 earned exactly $200000, with an average household gift of $554(from the study), that is 0.277% of income. Assuming everyone between $200000 - $100000 earned exactly $100000, with an average household gift of $7,087(from the study), that is 0.7087% of income Assuming everyone earning above $1,000000 earned exactly $10,000,000, with an average household gift of $17,072(from the study), that is 0.17072% of income. Is that generalisation that was made true?

Those who have everything to give, tend to give little. Those who have little to give, tend to give everything.

From the papers i quoted, it is not so convincing.

Sources: (1) https://www.theguardian.com/news/datablog/2010/sep/08/charitable-giving-country (2) https://public.wsu.edu/~taflinge/biology.html#preshuman (3) https://philanthropy.iupui.edu/files/research/giving_focused_on_meeting_needs_of_the_poor_july_2007.pdf