Hello Vicky, Baleno's weight is less compared to other cars in its segment because it has a brand new lightweight platform, which Suzuki has developed for its future lineup. It won't be a surprise if the next-gen Swift also comes with a lighter platform. In newer platforms across the globe, automakers are focusing on reducing the overall weight of the vehicle while increasing the structural strength. For eg, Kwid also uses an iteration of Renault-Nissan's global CMF platform, which indeed is a lightweight one and Kwid just weights mere 660 kilos. One more thing, being light weight doesn't not mean it will not do well in a crash test.
While developing the platform of the Baleno, Suzuki focused on reducing weight and improving the vehicles performance by concentrating on the underside of the car. For this Suzuki created simple shapes that resist deformation. While doing this the Baleno got a smoother shape with no bends.
This, in turn, helped Suzuki create a stronger structure that could do without reinforcements. Minimising these components, Suzuki was able to reduce the weight of the car by 15% or 100kg.
The new age materials that Baleno uses helps it achieve high strength with comparatively low volume of metal. Due to the low volume of metal, it achieves better performance (power-to-weight ratio) and better fuel efficiency too due to low kerb weight.