As the title says, this question came from me thinking about the main light and "medium" class tanks which both main Axis nations used before and during WW2. I am also aware of the reason both were stuck with the same tanks, even through the mid- to late war years, as both countries were located on smaller, narrower rocky strips of land compared to Germany, so there was not much of an incentive to build heavier armoured tanks (at least, at the beginning of the war). As the war dragged on and the Allied powers began shopping more (actual) medium tanks, I was wondering why Italy and Japan couldn't just effectively turn their tanks into what would be effectively be tank destroyers (or at least focus on building more of the existing ones), since most of their tank force were light and early-year medium tanks, and thus likely lighter and more nimble. Could this have to do with the smaller, less industrial capability of both countries, and the difficulty of ditching the production of lighter caliber guns and corresponding ammunition?
The Italy and Japanese did have some tanks with armor comparable to other tanks in ww2 and they did develop heavier armor and bigger guns for tanks. But there are a few problems. First off, let's talk about both countries' mid-war and late war tanks-
The Carro Armato P26/40 Heavy tank(which was really a medium tank, and was considered heavy by Italy because it was compared to their own light tanks), was a Italy design and only around 100~ was produced. They simply did not have the resources to make a lot of designs, and once they did, it was often considered too costly for them or they simply did not need it for their war strategies.
P40 Heavy tank was produced to keep the war effort in North Africa strong, as their tankettes(which was hugely out-classed by M3, crusaders, and other allied tanks there at the time) and light tanks simply didn't have the power and capacity to do anything. Moreover, Germany had helped and supplied Italy with tanks of their own. The P40 wasn't used by Italy at all, as the armistice had been signed before they could be delivered to a battle.
M41 with the 75mm was deployed in battle, but had a short service life and was based on the Stug.
The armor was comparable to the Sherman(50mm frontal armor angled at 50 degrees.), and sloped armor was derived for the T-34 after the Germans had captured some.
Many prototypes(such as the Carro Armato M Celere Sahariano, which is largely based on the Crusader) exist as well. But all of them were to slow to be designed, mass produced and delivered.
https://imgur.com/a/PbcTbOh (Image of P40)
More information for the Celere Sahariano
On the other hand, Japan.
Japan did not have many tanks, and most of them are lightly armored light tanks that could be penetrated by heavy machine gunfire. Most of their tanks were the Ha-go and some Chi-Ha, with some amphibious tanks like the Ka-mi. Both have small guns and weak armor.
The Japanese mostly focused around China and other small islands(The Philippines for example), which only have weak armies in which tanks are not an absolute necessity in the first place, and where naval and air superiority is needed to deliver fast punches. Both sides of the armies barely have any tanks to start with, and their infantry was weak enough so that expensive armored vehicles do not need to be produced to counter them as Japanese infantry was enough. In almost all Japanese armies, you would most likely see towed artillery, trucks, and some Ha-go tanks for motorized units.
Furthermore, Medium and heavy tanks need newly trained crews and a complex design, which makes repairs much longer and can hold up the army. Tanks with heavy armor like the Sherman Jumbo or the Tiger was not needed and was overkill when invading China and other islands, so fast and anti-infantry tanks were the main focus of Japanese tanks, not tanks with heavy armor and powerful guns often designed to face other tanks and armies equipped with anti-tank weaponry.
Could this have to do with the smaller, less industrial capability of both countries, and the difficulty of ditching the production of lighter caliber guns and corresponding ammunition?
While your saying does this heavily affects Italy, it does not apply to Japan. If they have the industrial capability to produce aircraft carriers and mass produce one of the most feared planes of the war(Zeros, Oscars, etc.), they certainly have the ability to produce tanks like the Americans or the Germans, but as said before, their military simply didn't need them, and their enemies and their battlegrounds would render medium and heavy tanks obsolete and a burden to infantry and the army in general, so yes for Italy, but a no for Japan.
I was wondering why Italy and Japan couldn't just effectively turn their tanks into what would be effectively be tank destroyers (or at least focus on building more of the existing ones)
They didn't need to. Italy signed the armistance in 1943 - no need for new weapons after that -, and most of the battles took place in Greece and Africa, where Germany helped them, and Italian medium tanks simply took too long to be designed and mass produced and probably limited to Italy's industrial capabilities. Italy did have some tank destroyers at the time(link), but they too were undergunned. Most of the up-gunned tank destroyers Italy had were blueprints and none of them were produced in the war, and most of the aduquate anti tank guns were towed.
Japanese did have projects, but only a handful of them actually had a prototype, but none of them was ever massed produced.
A caliber of up to 75 - 120mm was needed to defeat allied tanks at the time, and the Ha-go or Chi-ha chassis cannot support the weight of the gun or a turret mounting it. Developing a chassis would have taken too long and probably unworthy in where thier battles are mainly fought, as not all of the battles were fought against America all the time. And only a handful of battles against the US is ground battles, as most of them are naval. A new round would also need to be designed, but those are generally easy to make and produce, so shells aren't a problem here. Japanese already made tons of 37mm APHE, so they have the experience and the resources.
Though this does not go without saying that the Japanese, similarly to the Italians, do not have any medium/heavy tanks and many other prototypes.
The Chi-nu is the most produced Japanese medium tanks of ww2 and the only mass-produced Japanese tank that is at least somewhat comparable to allied and german tanks of the time.
Its armor is around the same as a Panzer 4 G model with added armor(around 50-60mm hull and UPF, not sloped), and it's long-barreled, high velocity 75 is enough to penetrate most Shermans frontally(except for the Jumbo) and even Kv-1s of that time.
However, similarly to the P40, only a small amount(144 total) was produced. They weren't used in combat and were only kept and used in the homeland(point above is justified with this fact, they had plenty of time to deliver those to different battlefields of their own) for the foreshadowed allied invasion, which never happened. Other prototypes, such as the Chi-To and the Chi-Ri was never produced.
The O-I super-heavy tank is also an interesting read on the topic of Japanese experimental tanks, similar to the Maus and the Ro-Go
Mass-produced:
Experimental and prototypes comparable:
All in all, Italy did not have the time and the industrial capability to crank out new medium or heavy tanks by 1943 and Japanese did not need highly complex and fuel-consuming medium or heavy tanks as their battlegrounds would present them a burden and not fit for them.