Index\vertex buffers memory sizes

Hi! Long story short:
I have two variants of the «same» scene (with landscape, particles, skeletal meshes, decals, lights, Static meshes, and many other actors), but there was optimized in a different way. When I gathering «memreport –full» via build of each scenes – I have some RHI stats that I don’t understand.

Screenshot_1 – Scene_1 - Index buffer memory not much different with Vertex buffer memory:

Screenshot_2 – Scene_2 - Index buffer memory in comparison with Vertex buffer memory is much smaller, and we have total memory usage economy.

Some optimisation work was done on Scene_1 (Skeletal\static mesh LODs triangles reduction, duplicating static meshes was replaced by HISM actors, some of the material optimization etc), but Index and Vertex buffers on this scene still near the same amount, while in scene_2 they are really different… I know that vertex buffer memory kind of storing info about vertices coordinates of all polygons, and index buffer help to store this data in a more universal way, without storing the same vertice coordinates in the different parameters. But in what way can I control this setup? How can they have such differences with nearly the same geometry in level?

** Is anyone know in what ways can I reach such a drastically different Index and Vertex buffers memories amount without significant scene appearance change?**
Sorry for bad English and thx in advance.