Nebius market cap is 57 billion.
Yandex is 65 billion.
Nebius focuses on AI infrastructure worldwide, Yandex focused on being a Google competitor in Russia ( if it survives it's DC's being destroyed).
Russias GDP is basically Italy.
Comparing GDP means nothing. Having larger GDP doesn't mean the country is stronger, it is just a number.
Also in this case, you are comparing not the GDP (actually produced goods and services) but a "market cap" which is a virtual number calculated by price of one share multiplied by share count. Yandex shares, I assume, cannot be traded internationally so the number means nothing. Yandex has a public report where one can find out their revenue.
Sometimes it's better to diversify, sometimes it's better to concentrate. There is no better decision to concentrate than to do it during the AI hype.
While Yandex could go bankrupt very soon ( 2-3/5 dc's closed, apps not working, robots not working, payments not working, maps show everyting in rivers and oceans, ...).
Nebius is currently going after their best growth driver. Can't blame them.