Having massive land gives you a much larger private market. With Max tech Theo you could buy out for like $2-4 billion cash (depending on acreage) and resell your mil to easy top 10.
I wasn't paying close attention but I think a couple were Tyr->Theo converts. You lose 14% of resources, but all your food is on the market so some buildings and small chunk of mil. Tyr gets you more land/less turns, Theo gets you the best deals (Dem with Max tech is next).

As a side note you will notice smaller top 10's go on a landgrab frenzy shortly before destock. This is to maximize private market.

Edit: If you choose to convert Tyranny to Theo usually works best for teching. As a techer you should be using the absolute minimum turns for grabbing and building. You can finish top 10 Tyr techer, but Express server seems to prefer Theo.