In addition to the answer of Tristan I would like to add few more points
The thrust in the rocket is equal to $T=\dot m V$ (Assuming the rocket nozzle is operating at its optimum condition)
The Thrust is a strong function of the exhaust velocity
$$V=\sqrt{\frac{2 \gamma R_{{}^{\circ}} T_{{}^{\circ}}}{(\gamma -1) \mu
}\left(1-\left(\frac{P_e}{P_c}\right){}^{\frac{\gamma -1}{\gamma
}}\right)}$$
This equation gives the exhaust velocity of the rocket
The exhaust velocity is a function of $\left(\frac{P_e}{P_c}\right){}^{\frac{\gamma -1}{\gamma }}$ and for vacuum the $P_e$ is almost equal to zero so the above term reduces to zero hence the exhaust velocity is maximum
For sealevel the above term does not reduces to zero so the exhaust velocity is less compared to that in the vacuum
Hence the thrust in the vacuum is more than that of in sea level (within atmosphere)