This was bugging me as well so I found an existing support case where someone asked this. It seems this was a change where they limited how small the traffic lights would get in 7.0 as it was thought they would sometimes be too small. There is a debug flag on the client that should let you get the old behavior for sizing. Steps pasted from that ticket below:
Change on chart minimum size has been added on 7.0 because we had issue with other chart rendering, and we limited minimum height for charts to be 100px.
However, there is a client debug flag that allows you to customize this minimum size.
To configure this debug flag,
1) Please open 'C:/Users/<USERNAME>/.dynaTrace/dynaTrace 7.0/dtclient.ini' with text editor.
2) Add following debug flag between "-vmargs" to "-Deof=eof" lines.
for example, -Dcom.dynatrace.diagnostics.client.chart.minHeight=20
3) Save dtclient.ini and restart AppMon Clilent if it is running.
It seems this change would be required on a client by client basis but hopefully it will help,