Why Sushmita never made it bigger?

Sushmita was popular as Miss Universe, still admired for her speaking skills, adopting girls, generally happy attitude. She has rarely had controversies.

She was part of hits like Biwi no1, Maine Pyar kyu kiya, Main Hoon Na.

Why she did not get much work or capitalise on success of her hit songs like Mast Mahaul, Dilbar etc?

I am also surprised she began to look aged or tired too soon. Like she does look kinda tired and worn out in some films she did later.

What happened to her? 

Just never had burning ambition & marches too much to her own drummer. 

because she lacked focus, discipline, professionalism and also she was too tall for the A-list heroes of her time 

Even Katrina and Deepika were too tall for heroes like Khans but they made it despite height difference.

I do not think height alone is a factor.

I am surprised she never worked with HR etc and took too many supporting or vamp roles like Sirf Tum, Bas itna sa khwab hai, Biwi no1 or typical sexually aggressive type roles or working woman in love with own boss or employees.

She was a terrible actress

Well Katrina and Ash were not good actresses either but they did well. Even Shilpa, Sonali were not good actors but did more work as lead heroines compared to Sush who was reduced to supporting roles.

Another factor might be her height . She was taller than 90% leading heroes of her time .

Because Sush was Sushmita Sen in all her movies..she never became the character..I have yet to watch Aarya though..and idk after she dated that weirdo Lalit Modi..I am kinda starting to doubt her intelligence as well..she picks the absolute worst person possible when it comes to men 😆