when i'm creating a button it become different size in different mobiles why...?

i'm new to the android when i'm creating the button it's become variation in different mobiles please help me. enter code here

    <LinearLayout android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="vertical">
        <RelativeLayout
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            android:layout_gravity="center_horizontal">
            <Button
                android:layout_width="wrap_content"
                android:gravity="center"
                android:width="300sp"
                android:layout_height="wrap_content"
                android:text="chennai to coimbatore"/>
        </RelativeLayout>
    </LinearLayout>