Canopy struts

Why were the struts of German planes so thick compared to Allied ones. Just look at the late Me 109 and the Fw 190?
Pilot view was certainly impeded.

In the case of the BF-109 the pilot's headrest was part of the canopy structure. No doubt it was intended to provide some measure of protection in case of a turnover mishap. The US preferred to make any such protection part of the fuselage structure. In the case of the P-35, there was enough room in the cockpit that the pilot could let down hid seat back and crawl out through the roomy baggage compartment.

The 109 had a very heavy canopy. It's not representative of all "German planes". FW190, Me262, Ju87 generally had no thicker frames than their allied counterparts.

On the 109 beginning with the G-1 all odd-numbered variants were pressurized and the heavier canopy framing was designed to carry those loads. For simplicity the unpressurized even-numbered variants had the same canopy framing. The aft canopy section (behind the opening canopy section) did not carry any pressurization load as the armored headrest was replaced by a pressure holding armor plate (with small clear inserts on the side edges) flush with the aft edge of the hinged canopy section.