^^^ He was talking about height, but that shouldn't limit the conversation, or make us blind to other factors. Pointing out that other undersized guys like Brees and Russell Wilson have an additional aspect (usually feet/mobility) that allows them to find better angles--either outside of the pocket or by pocket manipulation--to attack the middle (not to mention they're both mentally great QBs with decent to exceptional arm talent).
Keenum is too small to see/throw straight across the line, and not quick (or perhaps savvy?) enough to find the right angles at the right timing to hit middle-of the field routes with consistency. His arc and the relatively low speed of his throws force every throw to be timing and anticipation rather than read & react, which is tremendously dangerous when going over the middle.
It should also be remembered that, while Keenum gets credit for being a vet, it's not like he's started a very high amount of games throughout his career. In starts, he's what, a sophomore? He's come in for injured guys for stretches, but his 'experience' is overrated. There are still large parts of his game that suffer due to lack of reps, I'm sure. Attacking the middle with his TEs could be part of that.
To echo some, whenever Goff comes in, I would expect to see more slants and up-the-seam strikes. And more receivers on the injury report.