High cost of validating transactions is exactly analogous to heat dissipation in conductors that carry high currents.
And power is the product of voltage and current. P = V * I
High current is analogous to many transactions.
To move the same amount of money we can either:
1) use a small number of high value transactions
or
2) use a larger number of smaller value transactions.
2) is the situation with low voltage, high current power lines (high resistance, high heat loss, high fees)