It depends on the role of the vehicles, are they cavalry or mounted infantry?
If cavalry then the dismounts are an adjunct to the vehicle that provide scouting, man OPs, listening posts, defend the lager, lay ambushes, checkpoints, roadblocks, etc. They can also assist in maintaining the vehicles, preparing positions, including defences and camouflage, possibly even providing spare crews for the vehicles. The troops in the back would be Armoured Corps troopers rather than infantry and an integral part of the crew, assisting in the effective and proficient operation and employment of the vehicles. Smaller capacity vehicles are ideal for this and the ideal complement to gun cars or even tanks, highly effective at screening and providing security to supported units.
Mounted, including armoured, mechanised and motorised infantry are different in that the vehicle, and its crew, are an integral part of the embarked section. Rather than the dismounts supporting the vehicle, the vehicle supports the dismounts with the vehicle being a part of the section. Its all about the vehicle supporting the rest of the section, getting them where they need to go and supporting them. With the dismounts being more important there are more of them, usually six, seven, or eight, in one or two fire teams or bricks. This is your AIFV, MICV, usually tracked but occasionally wheeled with the vehicles being an integral part of the infantry battalions and the infantrymen being specially trained to operate with the vehicles. These battalions are also trained to work with armoured (tank) battalions / regiments.
Next you have your APCs, IMVs, etc. that provide armoured, or protected, lift for other arms, predominantly infantry but also engineers etc. This is where you have your capacity for a nine man section or even more , i.e. the original eleven man capacity of the M-113 or the even greater capacity of the AAAV-7. These are specialised units that transport other units, under armour, to where they need to go. The RAAC used to do this pretty well with their APC regiments and squadrons (reserve) providing lift for the RAR in Vietnam and reserve battalions back in Australia. Post Vietnam we moved to specialised mechanised, motorised, light and airborne infantry but are now moving to the concept of infantry supported by armoured cavalry.