The customer observed unbalanced traffic across two different physical interfaces, and BGP multipath was used to balance the traffic.

Physical interface: et-1/0/18:1, Enabled, Physical link is Up

 Traffic statistics:

  Input bytes :    1314787015856286           310848544 bps

  Output bytes :    1061705931591913            70394096 bps

  Input packets:       3472555690176              179090 pps    ## 217 Bytes

  Output packets:       1845649463068               28508 pps   ## 70394096/28508/8 = 308 Bytes per packet

Physical interface: et-1/0/19:1, Enabled, Physical link is Up

 Traffic statistics:

  Input bytes :     887293416263953            34884432 bps

  Output bytes :    2431436963492164          1068488584 bps

  Input packets:       1663874514693               27219 pps     ## 160 Bytes

  Output packets:       2015440142960               49962 pps   ## 1068488584/49962/8 = 2673 Bytes per packet

Based on the research, the main reason which contributed to traffic unbalanced was average packets length: 2673 vs 308, 8.7 times! Packets per second difference was small: 49962 vs 28508, 1.75 times.

It is an expected behavior due to some specific traffic flow carrying large packets.