They were already crammed like sardines, and for the hull front we are mostly talking about, sloping increases space in front of the driver rather than sacrificing it, like seen on the Porsche VK 4501 to VK 4502:
As for the French and Soviets, the only tanks where sloping elsewhere is really relevant would be the T-34 and FCM 36, but it's more complicated than "sloping reduced space".
In the case of FCM 36, weight was limited and it met all the specs, so sloping the sides of the hull and turret was just increasing the dimensions of the hull to improve the mass efficiency of the side armor. Everything that was needed to be packed in here was already packed. It actually managed to do that while increasing overall space for the crew compared to the other French light tanks.
Vickers tanks of the A9/A10/Valentine series and the Harry Hopkins followed the same diamond hull principle to try to improve armor mass efficiency for a fixed weight.
FCM 36 and A10. The other French light tanks had hulls mostly as wide as the FCM 36's hull is in front of the driver between the tracks, so the sloped side parts are a "bonus".
T-34 is more complicated because like most interwar tanks, it was progressively evolved from an existing design without completely reassessing the layout. The Soviets had not yet settled on a 3-man turret crew requirement, so the BT series and A-20 which started the T-34 development line still had 2 men. The 76mm gun was added during development, and it was understood only later that the turret was really too small. There was a reluctance from the Army and certain engineers to increase the turret ring diameter and (initially) reduce the slope of the side walls, even though the former was technically possible.
This was quickly changed however as after the first 38 production turrets, the T-34 got a turret widened by 160mm already, and eventually the hexagonal turret which reduced the slope of the side walls even more and increased height. The ring was not increased for a while because it had been tied to deep redesign programs (T-34M and T-43) which didn't enter production, and because of the chaotic situation for the Soviets up to 1943. The slope of the sponsons itself did not have a negative impact because the turret ring and basket were not extending there, and they just added extra space for fuel and ammo. Straight sponsons would not have materially improved crew space without an increase in the turret ring diameter.
Overall, turret side/sponson sloping rarely is the reason for "insufficient space". The insufficient space was already decided by the size of the turret ring and basket, and if it can be increased the sponsons and turret adapt, not the reverse. If there is a thing to blame, it's the army's or engineers' lack of understanding of how many people or how much space was needed for proper operation of a bulky 75mm+ caliber gun.
In fact the majority of interwar/early war tank design problems can be due to the following factors:
- incomprehension of the concept of the crew as a team and of the synergistic effects of a division of labor (results: overburdened commander, lack of observation devices or radio access for the loader when he is idle, operation of the driver and second hull guy as almost a separate team from the turret crew, resulting in overburdened driver and use of precious space by the second hull guy and a bow MG and/or radio instead of using the space for ammo or diesel fuel).
- overly restrictive list of targets for many tanks: fortifications, concentrations of infantry are not really included with the list so decent HE capability is not emphasized for many interwar tanks, even to relieve field guns and artillery from certain missions. Also occasionally results in some tanks having only machineguns even when a heavier armament would be possible.
- insufficient use of combined arms: great contributor to the existence of tanks with multiple turrets or MG turrets or rear turret MGs to cope with the lack of infantry support.
- excessive compartmentalization of tank developments in a country: results in good ideas from one company/design bureau not being applicated to other companies' designs when possible: very noticeable with the Christie suspension being almost exclusively considered in a convertible and internal form for certain tanks for a while in spite of the fact that it was a perfectly valid replacement for the fragile plunger/candle/needle suspensions in tracked-only or external form. Reluctance to try it on heavier vehicles even when it was later proved it could handle heavy weights. Only the Soviets really tried to apply it to all tank classes (T-46, BT series, T-29).
- tied to the previous problem: excessive reliance on evolving a design in a company rather than asking the questions "do we really need to do it like this? Do we really need to put this here?". This leads to certain fundamental flaws carrying over to a later design even when a major reassessment would have been warranted, eg. T-34 moving to a 76mm in a turret originally designed for a 45mm without many changes to the turret for a while, internal Christie enduring for so long, plunger-type suspensions carrying over to "fast" tanks even after it was desmonstrated they were too fragile for it (A6-->A7-->A14 in Britain).
