The difference between Undon noodles and Ramen noodles is that Undon are thick and Ramen are thin.
Undon noodles are thick while Ramen noodles are thin.
Undon noodles are thicker than Ramen noodles.
Noodles come in different textures. Japanese cuisine has become more famous around the world, but if your friend doesn't know the difference between the two just tell them that Undon is thicker. You can also provide more information by saying that Ramen is more yellowish in color while Undon is whiter. Kakejiru broth (has soy sauce, dashi and mirin) is used with Undon and Shoyu (a soy based broth) is associated with Ramen. They vary but both are very delicious!
Hope this helps!
いろいろな食感の麺がありますね。日本食は世界で知られるようになっていますが、この二つの違いを知らない人には「Undon is thicker（うどんの方が太い）」と伝えましょう。
より詳しく説明することもできます：
Ramen is more yellowish in color while Undon is whiter.
〔訳〕うどんは白くて、ラーメンは黄色っぽいです。
Kakejiru broth (has soy sauce, dashi and mirin) is used with Undon and Shoyu (a soy based broth) is associated with Ramen.
〔訳〕うどんにはかけ汁（しょうゆ、だし、みりん）が使われ、しょうゆ（しょうゆベースのスープ）はラーメンのイメージが強いです。
They vary but both are very delicious!
〔訳〕違いはありますけど、どちらも絶品です。
参考になるといいです！
"Normally udon is thick and ramen is thin" in this expression you are telling someone the obivious but with a slight caution. When you say normally your are saying that usually they are like that but there are some exceptions every now and then.
"Normally udon is thick and ramen is thin"
普通、うどんは太く、ラーメンは細い。
この表現では、明らかなことをちょっとした注意を加えて伝えています。 normallyとは、たいていそうなのであって、時には例外もあるということを伝えています。
Generally, udon noodles are thick and ramen noodles are thin
Normally udon noodles are chunkier than ramen noodles
Well, if it's fatter, you can call it 'thick'. Use the comparative form, and it's 'thicker'.
Or you can use the comparative form of 'chunky' and say 'chunkier'.
Udon noodles are thick and Ramen noodles are thin.
Udon and Ramen noodles are both made of wheat, but Udon noodles are thicker than Ramen noodles
Ramen noodles are thinner than Udon noodles.
When talking about the different types of noodles you can explain each one separately. Or you can compare and contrast the different noodles.
For example - you could explain that Udon noodles are made of wheat and are thick, but ramen noodles are much thinner.
The main difference between ramen noodles and udon noodles is the thickness
When explaining the difference between the noodles you can say that the size is the main factor. Ramen noodles are typically thin noodles with a deep yellow color from egg. Udon noodles are very thick and chewy made from wheat flour and water.
日本 
カナダ 
アメリカ合衆国 
イギリス 
オーストラリア 
