Technically it is possible. You don't need an equal or larger market cap to inflate something else 90%. You just need enough to dominate the trading volume.
As an extreme example, if bid/ask volume is exactly 1 (ie the ONLY trade volume consists of you and 1 other person trading a quantity of 1), then at the minimum, all you need is a bank roll of 1.9x the current unit price, with both you agreeing to the trade it for 1.9x, for the going price per unit to inflate 90%. And since market cap = price per unit × units outstanding, then the market cap also inflated 90%. If the units outstanding was 5000, and the unit price was $1, then the market cap increased $4,500 using only $1.90.
Look at trade volumes. Tether is massively bigger than bitcoin and ethereum combined.
Most people writing about USDT don't understand how arbitrage markets work, and don't understand that liquid markets are quick to resolve themselves (unlike ponzi schemes where you can hide the missing assets for a long time). Liquid markets will put quick pressure which is why these structures collapse faster (see MtGox)