One of the enduring and interesting mysteries of the Blitzkrieg campaigns in 1939-1942 is the low raw power of the then-victorious German tanks. Their numerical inferiority was most obvious in 1941 and 1942, but their qualitative inferiority (on spec sheets) was most obvious in France 1940 already.
Their numbers were inferior by a ratio of 1.5, but the armament and protection was a whole league lesser:
20 mm and 37 mm guns were typical, with few 75 mm stub guns (no better penetration than 37 mm) on Pzkpfw IV. Some German "tanks" were mere tankettes with machine guns only.
Meanwhile the opposing calibres included some low-powered 37 mm guns and some 75 mm stub guns, but mostly fine 37, 40* and 47 mm guns equal to or better than the German (and Czech-designed) 37 mm guns.
German maximum armour plate thickness was about 13-30 mm; bulletproofed. The Allies had but for one model only tanks with 30-60 mm in use - the Char B-1bis, Somua S35 and Matilda II tanks were shell-proofed and almost impenetrable to the German tank guns.
Many factors were combined to make up for this inferiority; training, personnel organisation (spare crews), maintenance, quicker refuelling, command and control by radio, better operational plan, quicker command system, better recourse allocation, better vision for crew members (especially the tank commander), better cooperation with infantry and artillery etc.
The message of all this is largely incompatible with modern perceptions of tanks: Nowadays it's (especially among laymen, but also among procurement officials, industry representatives, junior and senior military personnel) about the quality of the tank, and often about it's raw power.**
There's the seemingly ever-lasting perception that a penetrable tank is a poor tank, that the troops only deserve the best stuff (and "best" meaning mostly the most obvious qualities), much talk about the triad of tank power (firepower/protection/mobility, in varying order), and at times concerns about the ability to penetrate even the best-protected surfaces of 'threat tanks'.
Much money is spent on such things accordingly, but intense and realistic training (including training beyond the four days sound barrier of sleep discipline) with its unsexy expenses for fuel, spare parts, munitions and possibly repairs and compensations (for damaged civilian infrastructure, fences etc.) rarely seems to be at the centre of demands.
Army "readiness" is of public concerns at times, but did you ever see an article comparing your country's expenses for training per tank battalion with others'?*** How many publications do you know which state the time required to change a tank powerpack, the maintenance breaks required on administrative marches or the durability of track segments in km?****
"The Blitzkrieg Legend" by Karl-Heinz Frieser (this is the English edition)
*: The British 2 pdr gun lacked a HE shell as far as I know, though.
**: There are exceptions, of course. Technical experts pay much attention to detail and some armies pay attention to special skills such as crossing light bridges or driving through woodland).
***: The Americans and to some degree other anglophone countries have the "hollow force" scare to keep operational expenses up, but that's a more general concern.
****: I know Hilmes' and Ogorkiewicz' books do cover such 'hidden' qualities, and that's it.