Summers believed that it was only possible to come up with $225 billion in direct spending that had a high stimulus value. Further money could be targeted at tax cuts and aid to states, but it had a lower stimulus value. So there was a sense of diminishing returns here.
Now, that might have been wrong, but it's a fairly defensible position.
Read it at Mother JonesThe Real Reason Larry Summers Didn't Advocate a $1 Trillion Stimulus
by Kevin Drum