A full hybrid carries a battery a small fraction of the size found in an electric car, and that is a deliberate engineering choice rather than a cost compromise. The two batteries are asked to do entirely different jobs.
Two different jobs for a battery
An electric car's battery is an energy store. Its purpose is to hold enough for a journey, and it is discharged deeply over hours and recharged over hours.
A hybrid battery is a buffer. It captures energy from one braking event and releases it during the next acceleration, cycling many times an hour.
Sizing a buffer for range would add weight and cost that the layout gains nothing from, because the engine already provides the energy for distance.
Why small suits the duty cycle
Braking from moderate speed releases a limited quantity of energy, and a small pack is entirely capable of absorbing it.
What matters instead is power, meaning how quickly the pack can accept and release charge, since both happen over seconds rather than hours.
Cells optimised for power rather than capacity suit this well, and they can be packaged in a space that does not compromise the cabin or boot.
How the state of charge is managed
Hybrid control systems keep the battery within a narrow middle band, typically neither fully charged nor deeply discharged.
Staying in that band leaves headroom to absorb an unexpected braking event and reserve to assist an unexpected acceleration.
It also happens to be where cells age most slowly, so the operating strategy protects the hardware as a side effect.
Why weight is the real argument
A larger battery adds mass, and mass must be accelerated and braked constantly, which consumes some of the efficiency the hybrid exists to deliver.
The gain from additional storage falls away quickly once the pack can already absorb a typical braking event, while the weight penalty continues to grow.
The optimum therefore sits at a modest size, and hybrid packs have grown far less over the years than electric car packs have.
Why these packs tend to last
Shallow cycling within a narrow band is gentle treatment compared with repeatedly charging to full and running down to nearly empty.
Cooling is straightforward because the pack is small and its heat output modest, and steady temperature is the largest factor in how quickly cells age.
The combination is why hybrid batteries frequently remain serviceable for the life of the vehicle, contrary to the expectation that they would need replacing early.