So you are saying:

Simple linear regression with one variable (X1) versus multiple linear regression with X1 plus a group of insignificant covariates in the model?

The R^2 if not adjusted, will increase with additional variables in the model regardless of their significance. Adding more terms decreases the degrees of freedom in the calculations, so if you have the same term plus more terms, numbers inserted into the formula are not going to be the same in both scenarios.

P.S., You should run a model for fun to see this.