We cannot know for sure what Juniper refers to when stating "system capacity 80gps"
If we take the embedded 4 x 10G interface and we use them at full duplex then we get 80gbps. So it could refer to this fact. In this case adding line card would increase this number.
MPPS is much more informative as it refers to the actual capacity of the switch of processing packets.
From Cisco support forum:
For example, 2960-48PST-S is 13.3 Mpps.
The figure MPPS expresses the maximum number of frames per second that
can be processed by the device. It is not dependent on frame size but
clearly small frames require higher packet rates.
To give you an idea of what this number says:
smallest frames in Ethernet are 64 bytes in size, taking in account
the preamble (8 bytes) and the minimum inter-frame gap (the last two
counts roughly for 20.2 bytes) to fill a GE port in one direction you
need 1484560 frame per second.
10^9 / [(64+20.2)*8] where 8 is bits/byte.
So a number of 13.3 MPPS is equivalent to [((13.3 M * (64+20.2) * 8))
/ 10^9 = 8.95 / 2=4.47] 4.47 GE ports filled with smallest frames
On the other hand frames of max size 1518 bytes require 81264 fps to
fill a GE port in one direction.
So this number expresses the forwarding capability of the device.
A non blocking device with 48 GE ports would require 2 * 1484560 * 48
as MPPS or higher.
Note that the MX104 support frames up to 9192 bytes.
60mpps of 9192 bytes frames give you a max bandwidth of 4420 / 2210gbps (bidirectional / unidirectional)
60Mpps of 64 bytes frames give only 37/18,5 gbps.
If we are stuck with standard Ethernet MTU of 1500 we get something around 730/365gbps