Hi there Orhpancrow,
Marley Cole, in his book The New World Society, gives the figure for JWs in Germany in 1928 as 9,755. That number also originated from WTS records.
It does not seem plausible that the increase in Germany during the five years between 1928 and 1933 was over double. If the WTS numbers are to be believed, then you also have to believe that there were 12,245 Germans who converted during those 5 years - almost 2500 per year (using 22,000 as the number in 1933).
To address the reported 22 000 number in 1933.
Firstly are there numbers to support that value in the 1920's? (Before 1925 and the drop off?)
Yes. You can refer to Detlef Garbe Between Resistance and Martyrdom: Jehovah's Witnesses in the Third Reich – Page 47. Jehovah's Witnesses already numbered 20 000 in 1922.
So 1925 comes and goes and many in Germany are disappointed with new worlds no show and numbers decline. What happens between 1928 and 1933? The Great depression starts in earnest. Jehovah's Witness numbers always grow rapidly in times of uncertainty.
For comparable growth the Nazi party went from only 2.6% of the public vote in 1928 according to Encyclopaedia Britannica 2008 to a coalition MAJORITY in 1933! So I would argue that Watchtower growth in the face of German Fascism and the Great Depression is not just plausible but mandatory. Also as mentioned before the Watchtower is very particular in reporting accurate memorial attendance numbers. To fudge them would be something that I would find very hard to believe.
Finally I don't know where Marley Cole got the numbers because the 15 July 1927 Watchtower gives a breakdown of memorial attendance by individual congregations. The American and German numbers are comparable upon visual inspection. I have not tabulated the exact numbers but there's 15000+ attendees in Germany at least (it would seem), so I am puzzled by Cole's numbers.