A thick current carrying cable of radius R carries current I...

For inside part

$J = \dfrac{I}{\pi{R}^{2}}$

$∫ \overset{\to}{B} \cdot \overset{\to}{dl} = {\mu}_{0}{I}_{inclosed} \ldots \left(i\right)$

$B2\pir = {\mu}_{0}\left(\dfrac{I}{\pi{R}^{2}}\right) \times \pi{r}^{2}$

$B2\pir = {\mu}_{0}I\dfrac{{r}^{2}}{{R}^{2}}$

$B ∝ r$

For outside part

$∫ \overset{\to}{B} \cdot \overset{\to}{dl} = {\mu}_{0}{I}_{inclosed}$

$B ( 2\pir ) = {\mu}_{0}I$

$∴ B ∝ \dfrac{1}{r}$