For my point of view your image looks fine, I mean it's normal if road streets have thickness 3mm, the road quidance to be 10mm.
Anyway I can't understand what are you trying.....but if you like to play with quidance arrow try to change values to comand:
[2d]
guidancestrip_stretch=15
guidancestrip_stretch_forward=20
guidancestrip_thickness=0.4
guidancestrip_maxthickness=27
onewayarrowmodel=1
one_way_arrow_scale=50.0
[3d]
guidancearrow_first_size=1.1
guidancearrow_first_alpha=24
guidancearrow_second_size=0.8
guidancearrow_second_alpha=12
show_guidance_arrows=0
show_guidance_strip=1
guidance_arrow_head_width=5.0
guidance_arrow_head_length=4.0
guidancestrip_stretch=25
guidancestrip_stretch_forward=20
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