Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Fujifilm 18-135mm F3.5-5.6 R OIS is the longer of the two lenses at 98mm. The Fujifilm 18-55mm F2.8-4 R LM OIS with a length of 70mm is 28mm shorter. Besides being longer, the Fujifilm 18-135mm F3.5-5.6 R OIS also has a larger diameter of 76mm compared to Fujifilm 18-55mm F2.8-4 R LM OIS's 65mm diameter.
Weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Fujifilm 18-55mm F2.8-4 R LM OIS weighs 310g, 58% (180g) lighter than the Fujifilm 18-135mm F3.5-5.6 R OIS's weight of 490g.
Lens Mounts
Both Fujifilm 18-135mm F3.5-5.6 R OIS and Fujifilm 18-55mm F2.8-4 R LM OIS has the same Fujifilm X lens mount. Some of the latest released cameras that are compatible with the Fujifilm X lenses are Fujifilm X-H2S, Fujifilm X-T30 II and Fujifilm X-E4.
Focal Range
Fujifilm 18-135mm F3.5-5.6 R OIS has a focal range of 18-135mm and 7.5X zoom ratio which has a 35mm equivalent focal range of 27-202.5mm when used on a APS-C / DX format camera. Fujifilm 18-55mm F2.8-4 R LM OIS has a focal range of 18-55mm and 3.1X zoom ratio which has a 35mm equivalent focal range of 27-82.5mm when used on a APS-C / DX format camera.