microwave & toaster trip breaker

Generally NO! Its not a good idea to increase amp load capacity for many reasons.

1) A 20amp load on a 15 amp circuit may exceed the 80% rule. The wiring is a set gauge, even though most 15 (14g) & 20amp (12g) circuits may use (share) the wire gauge, it is important to know why (google search "wire gauges and there use").

2) Depending on the era and local the building was constructed... Building codes vary. Most MW and toasters need (should) be on a 20amp circuit. Most quality toasters are higher wattage users then MW. Their in the same energy use class as; Hair dryer, toaster ovens, blenders, food processers... etc. Some MFG's current designs require the appliance to be on and independent circuit back to the main panel.

You may have a weak panel breaker, though it would be a better idea to operate them on different circuits

(plugs not in phase with each other). Same holds through with refrigerators, should not be on same load as any of the mentioned due to current draw. Refers will pull upwards of 25amps during initial start up.