Factors:
Technique
Weight/Size
Quickness
Strength
Flexibility
Want-to/emotional maturity/motor
Gassed vs Fresh (depth, fitness,# of plays)
Understanding of role/scheme/etc.
IN GENERAL a senior is bigger, stronger, faster, has better weight and technique, has better chemistry, knows their job, and can play faster than can a freshman. Obviously there are exceptions, however across a roster a senior/upperclassman laden group is almost always more physical than a bunch of underclassmen for all the reasons above.
That said I noticed the D got pushed around less the second half. That tells me we came out flat...something we can't do given the youth of our roster. More want-to in 2nd half.
On O we lost Jordan Williams, and are playing Paula and W. Lay III at guards (3 guards ahead of them are hurt or slid to Jordan's tackle position).
Want-to, injuries/lack of depth, and youth are main issues imo. Somewhat interrelated.