Amplifying strain gauges on an arduino
Resistor \$R_G\$, between pins 8 and 9 of the INA125, sets the gain of the instrumentation amplifier, which is given by:

$$
G=4+\dfrac{60,000}{R_G}.
$$

If your output range is only one volt, and you want 5 V, reduce \$R_G\$ according to that equation, until G is 5 times higher.

If you want the details:

$$
k=\dfrac{G_{new}}{G_{old}}
$$$$
R_{Gnew}=\dfrac{15000·R_{Gold}}{15000 k - R_{Gold} + k R_{Gold}}
$$

The INA125's output is not rail to rail, so either use a supply > 5 V, to get a 5 V output range, or use a +5 V supply, and get an output range smaller than 5 V. Even if you adjust AREF of the Arduino to match your highest voltage, you will still lose range near the 0 V level.