It seems that there are some conflicting views when deciding to use a bold field input label or not.
“In this layout [with labels above the input fields], it’s advisable to use bold fonts for input field labels. This increases their visual weight and brings them to the foreground of the layout.”—Luke Wroblewski.
However, in an eye tracking study conducted by Luke Wroblewski, bold labels resulted in a 60% increase in saccade (fast movement of an eye) time to move from label to input field.
from 50ms without bold labels to 80ms with bold labels.
Reading bold labels is a little bit more difficult for users, so it’s preferable to use plain text labels. However, when using bold labels, you might want to style the input fields not to have heavy borders.
I’m my opinion, I wouldn't increase the font size of the field labels to increase the visual weight. Using bold field labels is a pretty common pattern in forms these days, and I’m sure big companies who employ this pattern, such as Google, have conducted their own research as to why they do it.