Is it harder to ride CR250 then Yz125 ??

Hi.
I have YZ125 1999 and my brother have CR250 1998.
Lately I am riding a lot on my brother's bike and I am getting tired very fast,
basically I am not in good shape ,but riding hard I can go on my YZ for 8 to 10 laps , and 4-5 laps riding my brother CR.
Can any one tell me if it's regular to get tired on 250 then on 125? May be its something else? Suspension? Any one has any Idea??

I sent my YZ cylinder for replating, so I am going to race on my brother's bike next month, and for now its looks like I want be able to finish the race (12 lap's beginner level), so please help!
:ugg:

Thanks

Thanks Cactus Rider, Your words make since, the track I am practice on is very tight, the longest straight is about 100'-120 ' , so this is probably the cause to the different between the 125 and the 250.
As you can see I am from Israel, and motocross is basically out of law (long story...lots of bureaucracy) so we have something like 40 to 50 motocross rider that racing regularly.
We have 5 levels:
Beginners - you can use any bike any size.
125cc - any 125 2 strokes or 250 4-strokes bike and rider must have
been at least 2 time on podium at the beginners level.
250cc - just as 125cc but bike is 250 for 2 strokes and 500 for 4
strokes.
Open class - any bike , and rider as 125.
Senior - as open class but rider age must be over 32.
I know it sound to you guys funny but this is the best we can do as long as it is out of law hear .
It's hard to be motocrosser hear :mad:
So now you can see that I can use the 250 or 125 in the beginners level ,the track is going to be less tighter then the one I practice , so I should expect the 250 to be easier to ride out there .
Next week I will be training on that race track and I will try the 250.
Nikki, you are right, in fact lately I am training (running, biking, pushup and so..) more then I riding the bike , It is my first race and I am not expecting much , just finish it .. :mad: