You are making the mistake of assuming that Boudicca's army depended on the complex supply system of a Roman legion. It didn't. It was an army raised from locals fighiting on its own territory, and thus could easily supply itself from the same stores that supplied the men when they were working the fields rather than fighting.

Even if it did, the Germans nor the French were not able to maintain more than 100,000 soldiers in their own territory.

Northern Europe that raised militias in the 14th century - such as Sweden, Denmark or Norway, mostly had battles with 300-500 soldiers that barely could be fed, that often got a bit awkward if they fought over a bridge.

That the decentralized tribes a millenium earlier could mount larger armies is absurd.

Aside from lacking a source, this claim makes the fundamental mistake of thinking that the number of soldiers fielded at any time is simply a function of the agricultural output and population of a region. It's simply not true. A simple counter-example: 17th century France had a population of 20 million, and was fielding armies of around 20000 men; at the end of the 18th century, the population had risen to 28 million (+40%), yet France with its levee en masse was capable of fielding hundreds of thousands of men (much more than a 40% increase). The economic, social, political, technological and cultural context all play a role in army size: what kind of authority do you have to recruit men? how can you convince them to fight? are they useful and useable in such large numbers? do you need to pay them? will they be faithful? etc. etc.

The largest issue here is the agricultural expansions and medical improvements in the 18th century - lessening the need of manpower in the fields and opening up for conscription and the industrial revolution in the 19th century.

Neither medical nor agricultural revolutions had happened in 60 AD, they were a lot more primitive and with a lot less population than what existed in Europe a millenia later.

This is simply wrong. Ancient sources are fond of giving figures, and rarely just state "many" when recounting battles for which accounts had survived. Dio's account of Boudicca's rebellion clearly states 230000 men in her army, for example (which of course can be challenged as a figure, but still).

Often when numbers were given, it was other words for "all", as for isntance Hebrew sources always numbers that were multiples of 13 as to state "all" (the common statement of all was all the people of all tribes, that usually was the number 13 * 13 * 1000 = 169,000).

"First today"? historians have critically analyzed ancient sources for at the very least a century; troop numbers have hardly started to be criticized "first today".

Difference between 19th century discussions over the validity of sources and modern analysis of economic and agricultural output varies greatly.